The present report summarizes the pertinent clinical and pathologic findings in 165 cases of leukemia in atomic bomb exposed victims autopsied during the period 1949 to 1962 at ABCC in Hiroshima and Nagasaki, Japan. Significant parenchymal hemorrhage occurred most often in acute myelogenous leukemia and was markedly increased in patients dying with high terminal white blood cell counts. Possible mechanisms involved in the pathogenesis of cerebral hemorrhage in leukemia are discussed. Subarachnoid hemorrhage and subdural hematoma were not related to leukocytosis but appeared to be influenced by marked thrombocytopenia. Leukemic infiltrates of a diffuse nature involving the meninges were paradoxically increased in patients receiving adequate chemotherapy. Meningeal tumors did not show this peculiar relationship to therapy and were not found in association with lymphatic leukemia. Infections involving the central nervous system were confined to patients receiving chemotherapy including steroids. 39 references, 3 figures, 4 tables.