Chronic lymphocytic leukemia (CLL) is technically both a leukemia and a lymphoma. The World Health Organization classifies CLL and its counterpart, small lymphocytic lymphoma (SLL), as two manifestations of the same disease. The difference comes down to where the cancer cells accumulate: primarily in the blood (leukemia) or primarily in the lymph nodes (lymphoma). The underlying biology is identical.
Why CLL Has Two Names
CLL and SLL involve the same type of abnormal white blood cell, a slow-growing B lymphocyte that multiplies out of control. When these cells build up mainly in the bloodstream and bone marrow, the disease is called CLL. When they collect primarily in the lymph nodes, spleen, or other tissues outside the blood, it’s called SLL. Many patients have some overlap, with cancer cells in both locations.
The formal dividing line is a blood count. A CLL diagnosis requires at least 5 billion B lymphocytes per liter of blood, sustained for at least three months. If the count falls below that threshold but you have swollen lymph nodes from the same abnormal cells, the diagnosis is SLL. In practice, doctors treat them the same way and use the combined label “CLL/SLL” in clinical guidelines.
How It’s Diagnosed
CLL is often caught by accident. A routine blood test shows an unusually high white blood cell count, and follow-up testing confirms the cells are clonal (all copies of the same abnormal cell). A test called flow cytometry identifies the characteristic protein markers on the cell surface. CLL cells carry a specific combination of markers that distinguishes them from other blood cancers.
SLL follows a different path to diagnosis. Because the blood count may look relatively normal, the disease usually comes to attention when a doctor notices enlarged lymph nodes during a physical exam or imaging scan. Confirming SLL typically requires a lymph node biopsy, where a pathologist examines the tissue under a microscope and runs the same flow cytometry tests.
Who Gets CLL
CLL is the most common type of leukemia. According to the most recent data from the U.S. SEER database, there are about 4.6 new cases per 100,000 people each year. For 2024, an estimated 20,700 new cases were expected in the United States, representing roughly 1% of all new cancers. The median age at diagnosis is 70, making it predominantly a disease of older adults. It’s roughly twice as common in men as in women.
Symptoms to Recognize
Many people with early CLL have no symptoms at all and learn about their diagnosis only through blood work. When symptoms do appear, they tend to develop gradually. Swollen lymph nodes in the neck, armpits, or groin are common. Fatigue that doesn’t improve with rest is frequent. Some people experience what oncologists call “B symptoms”: drenching night sweats, unintentional weight loss of more than 10% of body weight, and fevers without an obvious infection. An enlarged spleen can cause a feeling of fullness or discomfort on the left side of the abdomen.
Because CLL crowds out healthy immune cells over time, frequent or unusual infections can be an early sign that the disease is progressing.
How Doctors Assess Severity
Two staging systems help predict how CLL will behave. In the United States, the Rai system groups patients into three risk categories. Low risk means lymphocyte counts are elevated but there are no other complications. Intermediate risk adds enlarged lymph nodes, spleen, or liver. High risk means the disease has started to crowd out normal blood cell production, causing anemia or low platelet counts.
In Europe, the Binet system uses a similar logic, staging patients based on how many areas of the body have enlarged lymph nodes and whether blood counts have dropped.
Beyond staging, genetic testing of the cancer cells provides important information about prognosis. Certain chromosomal changes carry very different outlooks. A deletion on chromosome 13 (called del(13q)), when it’s the only abnormality, is associated with a favorable prognosis and slower disease course. On the other hand, deletions on chromosome 17 (del(17p)) or chromosome 11 (del(11q)) signal more aggressive disease that may need earlier or more intensive treatment. These genetic markers often influence which therapy a doctor recommends.
Treatment Approach
Early-stage CLL that isn’t causing symptoms often doesn’t need treatment right away. This strategy, called “watch and wait,” involves regular blood tests and check-ups. It can feel counterintuitive to monitor a cancer without treating it, but starting therapy too early hasn’t been shown to improve outcomes for slow-growing disease.
When treatment becomes necessary, the landscape has shifted dramatically in the last decade. Older chemotherapy-based regimens have largely been replaced by targeted therapies that block specific signals the cancer cells need to survive. The most widely used class works by blocking an enzyme called Bruton tyrosine kinase, which drives the growth of CLL cells. These drugs are taken as daily pills and can keep the disease controlled for years. Second-generation versions of these drugs have been developed to reduce side effects while maintaining effectiveness.
Another major treatment option works by directly triggering the cancer cells to self-destruct. This approach is often combined with an antibody that targets a protein on the surface of B cells, and the combination can achieve deep remissions. Unlike some targeted therapies that are taken indefinitely, this combination is given for a fixed duration, typically about 12 months.
Because CLL and SLL are the same disease at the cellular level, the treatment options are identical regardless of which label applies to your diagnosis.
Richter Transformation
In a small percentage of cases, CLL transforms into a much more aggressive lymphoma. This is called Richter transformation, and it occurs in roughly 2 to 10% of CLL patients, at a rate of about 0.5 to 1% per year. In 95 to 99% of these cases, the disease becomes diffuse large B-cell lymphoma, a fast-growing cancer that requires prompt, intensive treatment. Signs of transformation include rapidly growing lymph nodes, sudden worsening of B symptoms, and sharply rising blood markers. Richter transformation can occur in patients who have never been treated as well as those who have had prior therapy, though it’s more common after treatment.