Post-mortem clinical and pathological study of 18 cases of central nervous system leukemia showed that this complication occurred mostly in chronic myelogenous leukemia (38.8%). No diagnostic criteria was found. The great majority of signs and symptoms were related to either d sturbances of the mental status or cranial nerves dysfunction. Cerebrospinal fluid may be found normal. CNS involvement may occur at any time during the course of systemic leukemia, when the disease is under apparently good therapeutic control as well as during relapse. Pathological findings in order of decre sing frequency were: parenchymal hemorrhage (61%); subarachnoid hemorrhage (55%); meningeal infiltrates (44%); leukostasis (28%); edema and herniation (28%); parenchymal infiltrates (22%); ischemic infarcts (17%); progressive multifocal leucoencephalopathy (10%); calcifications (5%); meningitis (5%). Total survival time ranged from 8 to 1980 days a median of 300 days. Survival time after CNS involvement ranged from 1 to 180 days with a median of 21 days.