• Owing to improved therapy and lengthened life span, the incidence of neuromeningeal involvement in leukemia is increasing. Careful examination of the cerebrospinal fluid (CSF) is important for an early diagnosis. Among the available techniques, the use of cytocentrifugation enables us to demonstrate central nervous system leukemia even if the white blood cell count in the CSF is under 10/cu mm. We describe the results obtained by examining 631 CSF samples from 87 patients affected by acute leukemia; central nervous system luekemia was found in 22.7% of the patients suffering from acute lymphocytic leukemia and in 6.4% of those with acute nonlymphocytic leukemia (ANLL), but this ratio is higher in ANLL compared with the survival as measured in months (a ratio of 2.0 in ANLL compared with 0.50 in acute lymphocytic leukemia). A "leukemic" CSF was found in 51.5% of prophylactically treated patients and in 73.1% of the untreated ones.