The specific cause of leukemia is still not known. Scientists suspect that vital, genetic enviromental, or immunologic factors may involved.

Some viruses cause leukemia in animals, but in humans, viruses cause only one rare type of leukemia. Even if a virus is involved, leukemia is not contagious. There is no increased incidence of leukemia among people (friends, family, care givers) who have close contact with leukemia patients.

There may be a genetic predisposition to leukemia. There are rare families where people born with Chromosomes damage may have genes that increase their chances of developing leukemia.

Enviromental factors such as high-dose radiation and exposure to certain toxic chemicals have been directly related to leukemia, but only in extreme cases, such as atomic bomb survivors in Nagasaki and Hiroshima or industrial workers exposed to benzene. Controlled exposure to ordinary x-rays, like chest x-rays, is not believed to be dangerous.

Finally, people with immune system deficiencies appear to be at more risk for cancer because of the body's decreased ability to resist foreing cells. There is evidence that patients treated for other types of cancer with some types of chemotherapy and/or high-dose radiation therapy may develop leukemia later on.

All of these factors may account for a small number of the people with leukemia. Again, among the majority of people, the cause is not known.