Urinary tract infections are the most common cause of blood in urine across all age groups. The medical term for blood in urine is hematuria, and it can be visible to the naked eye (turning urine pink, red, or cola-colored) or detectable only under a microscope. While a UTI is the leading cause, the full list of possibilities ranges from completely harmless to serious, and the likely explanation shifts depending on your age and sex.
How UTIs Cause Bleeding
A bacterial infection anywhere in the urinary system can produce blood in urine. This includes infections of the bladder (cystitis), the kidneys (pyelonephritis), or the urethra. When bacteria invade and inflame the lining of these structures, the tissue becomes swollen and fragile enough to bleed. UTIs are the top cause of visible blood in children’s urine and remain the single most frequent explanation in adult women, who are far more prone to bladder infections than men due to a shorter urethra.
Most UTIs also cause burning during urination, a frequent urgent need to go, and sometimes cloudy or strong-smelling urine. Blood from a UTI typically clears once the infection is treated. If you notice blood without any of those classic symptoms, a different cause becomes more likely.
Kidney and Bladder Stones
Stones that form in the kidneys or travel into the ureters (the tubes connecting kidneys to the bladder) are another very common source of blood in urine. The stones scrape against the lining of the urinary tract as they move, causing microscopic or visible bleeding. People with ureteral stones usually experience intense pain in the lower back or side, sometimes with vomiting. Some stones, however, cause no pain at all and are discovered only because blood shows up on a urine test.
Stone-related hematuria generally resolves once the stone passes or is treated. The pain tends to come in waves and may shift location as the stone moves downward through the urinary tract.
Enlarged Prostate in Men
In middle-aged and older men, an enlarged prostate gland is a frequent cause. The prostate sits just below the bladder and surrounds the urethra, so when it grows, it can compress and irritate the urinary tract. About 11% of men with benign prostate enlargement develop blood in their urine. The bleeding happens because the growing tissue develops new blood vessels that are fragile and prone to rupture. The average age at presentation is around 65.
An enlarged prostate also causes a weak urine stream, difficulty starting urination, and the feeling that the bladder isn’t fully empty. These symptoms overlap with prostate cancer, so doctors typically investigate further when blood appears alongside them.
When Blood Could Signal Cancer
Painless blood in urine, especially visible blood, raises concern about cancers of the bladder, kidney, or prostate. The overall picture, though, is reassuring for most people. A large study published in The Journal of Urology found that the three-year cancer incidence among people with blood detected on a urine test was 0.68%. In adults under 40, the rate was below 0.5% regardless of how much blood was present.
Risk rises meaningfully after age 40, particularly in men. Cancer was found in more than 1% of men over 40 who had moderate amounts of blood on microscopy, and exceeded 3% in men over 40 with higher levels. Being older than 40 was the single strongest predictor, increasing cancer risk roughly 17-fold compared to younger adults. Male sex raised the odds about fivefold. Among patients specifically referred to a urologist for evaluation of microscopic blood, cancer was found in 5% to 13% over one to two years of follow-up.
The key detail: cancer-related bleeding is usually painless. If blood appears without burning, pain, or signs of infection, doctors are more likely to order imaging or a scope of the bladder to rule out a tumor.
Exercise-Induced Hematuria
Intense physical activity, particularly long-distance running, can temporarily cause blood in urine without any underlying disease. Two mechanisms are at play. First, during hard exercise the body diverts blood away from the kidneys, and the resulting changes in blood flow and pressure can damage small vessels. Second, in runners specifically, the back wall of an empty or near-empty bladder can repeatedly slap against the bladder base with each stride, creating bruises that bleed.
Contact sports like football and boxing can also cause bleeding from direct trauma to the kidneys. In all these cases, the blood typically disappears within 24 to 48 hours of rest. If it doesn’t clear in that window, something else is going on.
Foods and Medications That Mimic Blood
Not all red urine actually contains blood. Beets, blackberries, and rhubarb can temporarily turn urine pink or red, a phenomenon sometimes called “beeturia.” Certain medications, including senna-based laxatives and some older psychiatric drugs, also change urine color to red or orange. These false alarms are harmless, but it’s impossible to tell visually whether the color comes from food or actual blood. A simple urine test can distinguish the two.
How Doctors Determine the Cause
The standard definition of microscopic hematuria is more than 3 red blood cells per high-power field on a single properly collected urine sample. That threshold, established by the American Urological Association, is what triggers further workup even when urine looks normal to the eye.
Doctors use a risk stratification system based on the amount of blood, your age, and your sex to decide how aggressively to investigate. A 25-year-old woman with a small amount of microscopic blood and UTI symptoms will usually just be treated for the infection. A 55-year-old man with visible painless blood will typically get imaging of the kidneys and a cystoscopy (a thin camera inserted into the bladder) to check for tumors or stones.
Other causes that come up during evaluation include kidney disease (particularly a condition where the kidney’s filtering units become inflamed), inherited disorders like sickle cell disease, and certain blood-thinning medications that make normal tiny bleeds more visible. In a meaningful number of cases, no cause is ever identified, and the blood resolves on its own.