Cancer, leukemia: Leukemia is a cancer of the white blood cells. Leukemia's are grouped by how quickly the disease develops (acute or chronic) as well as by the type of blood cell that is affected. People with leukemia are at significantly increased risk for developing infections, anemia, and bleeding. Diagnosis of leukemia is supported by findings of the medical history and examination, and examining blood under a microscope. Leukemia cells can be detected and further classified with a bone marrow aspiration and/or biopsy. Most patients with leukemia are treated with chemotherapy. Some patients also may have bone marrow or stem cell transplantation.