Although leukemia refers to a malignancy of white cells, red cells also have a cancerous form, called erythroleukemia (literally "red leukemia"). Erythroleukemia appears much less frequently than cancers of the white cells and often involves abnormalities of white cells and platelets, too. Researchers, who have observed that the several different subcategories of white cells can each develop into a particular kind of leukemia, believe that both red and white cells may share a common ancestor in the bone marrow. This common ancestor, the stem cell, may explain why every leukemia, no matter which cell line it affects primarily, generally affects all of the other blood cells to a lesser degree.

The lymph cells give rise to lymphatic leukemias that directly affect the immune system. Granulocytes (or myelocytes), the proper medical name for the white cell group that engulfs bacteria, give rise to granulocytic (or myelocytic) leukemia. Among the subgroups of white cells, the eosino-phils that are associated with allergy and the monocytes that defend against such chronic infections as tuberculosis can each develop a corresponding leukemia, although these diseases are much more rare than simple granulocytic leukemia.

The leukemias are also classified according to rate of progression of the disease. Those that progress rapidly are known as acute, while those that are slower in their course are called chronic. This method of classification, although not perfect, is quite useful for diagnosis, treatment, and prognosis. Acute lymphocytic leukemia, for example, is a very different disease from its slower counterpart, chronic lymphocytic leukemia. Each has very different implications for patients, family, and doctor. Patients with acute lymphocytic leukemia, many of them young children, may paradoxically enjoy a much better prognosis than patients with the chronic form. In fact, they now have an excellent chance of being cured of their disease.

On the other hand, many chronic leukemias may cause few if any symptoms and require little or no treatment for many years. At present, about 50 percent of all leukemias are acute, 20 percent are chronic granulocytic, and 30 percent are chronic lymphocytic. Late in its course, a chronic leukemia may transform itself into a more rapidly progressive acute leukemia.