Hairy cell leukemia is a very rare cancer that causes the body to make too many white blood cells (lymphocytes). Hairy cell leukemia has this name because the lymphocytes look hairy under a microscope.

When leukemia cells build up in the blood and bone marrow, there is less room for healthy blood cells. This can cause infections, anemia, and easy bleeding.

Symptoms of hairy cell leukemia (HCL) can include weakness, fever, weight loss, bruising easily, and repeated infections. The spleen may become swollen and painful.

HCL may grow very slowly or it may not get worse. So it often does not need to be treated right away. It is more common in men than in women. It occurs more often in adults in their 50s and is not seen in children or teens.

HCL is considered a chronic disease, because it does not ever go away completely. But treatment can keep symptoms away for long periods of time. And if a person who has HCL is under the care of a doctor, he or she can have a normal life expectancy.