Hairy cell leukemia is a very rare cancer that causes the body to make too many white blood cells (lymphocytes). These lymphocytes don't develop normally and become leukemia cells. Hairy cell leukemia has this name because the leukemia cells look hairy under a microscope.

When leukemia cells build up in the blood, bone marrow, and spleen, 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.

HCL doesn't ever go away completely. But treatment can keep symptoms away for long periods of time. When under the care of a doctor, a person who has HCL can have a normal life expectancy.