Chronic leukemia is a slow-growing cancer of the white blood cells. Unlike acute leukemia, where immature, poorly formed cells multiply rapidly and cause symptoms within weeks, chronic leukemia involves partially mature white blood cells that accumulate gradually over months or years. Many people are diagnosed without any symptoms at all, after a routine blood test reveals an abnormally high white blood cell count.
There are two main types: chronic lymphocytic leukemia (CLL), which affects a type of white blood cell called lymphocytes, and chronic myeloid leukemia (CML), which affects myeloid cells, the precursors to several types of blood cells. Both are cancers of the bone marrow, but they behave differently, are treated differently, and have distinct outlooks.
How Chronic Leukemia Differs From Acute Leukemia
The core difference comes down to how mature the cancerous cells are. In acute leukemia, the bone marrow churns out immature cells called blasts that can’t function as normal blood cells. These blasts make up more than 20% of the bone marrow or bloodstream, crowding out healthy cells quickly and causing severe symptoms within days or weeks.
In chronic leukemia, the cancerous cells are partially mature. They look more like normal white blood cells under a microscope and may even carry out some of their usual functions, at least early on. Because these cells accumulate slowly rather than explosively, chronic leukemia often goes unnoticed for years. The blast count stays below 20%, and the body compensates for a long time before problems surface.
The Two Main Types
Chronic Lymphocytic Leukemia (CLL)
CLL is the more common of the two. It starts in B lymphocytes, a type of white blood cell that normally helps fight infections by producing antibodies. In CLL, these B cells become cancerous and accumulate in the blood, bone marrow, liver, spleen, and lymph nodes. The cells don’t die off on their normal schedule, so they pile up over time rather than reproducing especially fast. CLL tends to be diagnosed in older adults, and its five-year relative survival rate is about 90%, reflecting how slowly it progresses in most people.
Chronic Myeloid Leukemia (CML)
CML affects myeloid cells, which are the precursors to red blood cells, platelets, and most white blood cells. Nearly all CML cases are driven by a specific genetic accident: a swap of DNA between chromosomes 9 and 22, creating what’s known as the Philadelphia chromosome. This swap fuses two genes together, producing an abnormal protein that acts like a stuck “on” switch for cell growth. The protein activates multiple signaling pathways that tell myeloid cells to keep dividing, while also blocking the normal self-destruct mechanism that clears out damaged cells. The result is a steady, uncontrolled buildup of myeloid cells, along with an enlarged spleen.
Symptoms and How It’s Found
Most people with chronic leukemia feel completely fine when they’re diagnosed. The discovery typically happens during a routine complete blood count ordered for an unrelated reason, when the lab flags an abnormally high number of white blood cells. Only about 5 to 10% of CLL patients have noticeable symptoms at diagnosis.
When symptoms do appear, they tend to develop gradually:
- Swollen lymph nodes: Painless lumps in the neck, armpits, or groin are the most common physical sign of CLL, found in 50 to 90% of patients at some point.
- Enlarged spleen: Present in 25 to 55% of CLL cases, this can cause a feeling of fullness or discomfort on the left side of the abdomen, along with early satiety (feeling full after eating very little).
- Fatigue: Persistent, unusual tiredness that doesn’t improve with rest.
- Night sweats and fevers: Drenching sweats during sleep or fevers above 100.5°F lasting more than two weeks with no sign of infection.
- Unintentional weight loss: Losing 10% or more of body weight over six months without trying.
- Frequent infections: Because the cancerous white blood cells don’t fight infection effectively, you may get sick more often or have infections that linger.
CML shares many of these symptoms but is more likely to cause an enlarged spleen early on, sometimes noticeably so.
How CML Progresses Through Three Phases
CML has a well-defined progression pattern that doctors track using the percentage of blast cells in the blood and bone marrow.
In the chronic phase, blasts make up less than 10% of the blood or bone marrow. Most people are diagnosed in this phase, and the disease is easiest to control here. Without effective treatment, CML eventually moves into the accelerated phase, where blasts rise to 10 to 20%, platelet counts may drop, and the disease becomes harder to manage. The final stage, blast phase (sometimes called blast crisis), looks and behaves more like acute leukemia: blasts reach 20% or higher, and cancer cells may spread to organs beyond the blood and bone marrow. Modern treatment has made progression beyond the chronic phase far less common than it once was.
How Chronic Leukemia Is Diagnosed
A diagnosis usually begins with that unexpected blood test. For CLL, the key finding is an elevated count of clonal B lymphocytes. The current diagnostic threshold is a clonal B-cell count above 5 × 10⁹/L (5,000 per microliter of blood). Below that threshold, the condition is classified as a precursor state rather than CLL itself. Doctors confirm the diagnosis by examining the blood cells more closely with a technique called flow cytometry, which identifies the specific proteins on the cell surface that mark them as CLL cells.
For CML, the hallmark is the Philadelphia chromosome. A blood or bone marrow sample is tested for this chromosomal swap, and if the fusion gene is found, the diagnosis is essentially confirmed. Additional bone marrow biopsies help determine which phase the disease is in.
Treatment for CML
CML treatment was transformed by the development of targeted pills called tyrosine kinase inhibitors (TKIs). These drugs work by blocking the abnormal protein created by the Philadelphia chromosome. The protein normally fuels cell growth by acting as a constantly active enzyme. TKIs fit into the enzyme’s active site like a key in a lock, preventing it from sending growth signals.
Four TKIs are approved as first-line treatment for chronic phase CML: imatinib, dasatinib, nilotinib, and bosutinib. Imatinib, the first to be developed, was a landmark in cancer treatment because it turned CML from a disease with a poor prognosis into one that most people can live with for decades. The newer options were developed to work in cases where imatinib isn’t effective enough or causes intolerable side effects. All are taken as daily oral pills, and most people stay on treatment long-term, though some patients who achieve a deep and sustained response may be able to stop under careful monitoring.
Treatment for CLL
CLL is managed very differently. Because it progresses so slowly, many people don’t need treatment right away, or even for years. The standard approach for early, asymptomatic CLL is active surveillance: regular blood tests and physical exams to watch for signs that the disease is worsening. This isn’t a lesser form of care. It’s a deliberate strategy based on evidence that treating too early doesn’t improve outcomes and only exposes you to side effects you don’t yet need.
Your care team will monitor for specific changes that signal it’s time to start treatment: rising leukemia cell counts, dropping red blood cell or platelet counts, growing lymph nodes, or an enlarging spleen or liver. Symptoms like worsening fatigue, night sweats, fever, frequent infections, unexplained weight loss, or new lumps are also triggers to reassess.
When treatment is needed, CLL is typically treated with targeted therapies that interfere with the survival signals keeping the cancerous B cells alive, or with antibody-based treatments that help the immune system recognize and destroy them. The specific approach depends on the genetic features of the cancer cells and your overall health.
Living With Chronic Leukemia
The prognosis for chronic leukemia has improved dramatically. CLL has a five-year relative survival rate of about 90%, and many people live with it for 15 to 20 years or longer, especially those diagnosed at an early stage. CML, once a disease with a median survival of three to five years, now has a near-normal life expectancy for patients who respond well to TKI therapy.
Both types require ongoing monitoring, even when you feel well. For CML, this means periodic blood tests to ensure the fusion gene remains suppressed. For CLL on active surveillance, it means regular check-ins to catch any progression early. The slow nature of chronic leukemia means the relationship with your hematologist is a long one, but for most people, the disease is something they live with rather than something that defines their daily life.