The frequency of central nervous system (CNS) leukemia was studied in patients aged 15-59 with acute leukemia, who had received induction treatment in the years 1971-1986. Twelve out of 103 patients with acute lymphoblastic leukemia (ALL) developed CNS leukemia in spite of prophylaxis consisting of intrathecal methotrexate. Ten out of 217 patients with acute myelogenous leukemia (AML) developed CNS leukemia. None had been given preventive treatment. Leukemic blasts with either M4 or M5 morphology appeared to increase the risk of CNS relapse. Treatment was adjusted to the clinical problem of each patient, but always included intrathecal methotrexate. Median survival after a diagnosis of CNS leukemia was 8 and 6 months in ALL and AML respectively, with bone marrow failure due to hematologic relapse as the leading cause of death. CNS leukemia, if properly treated, does probably not shorten survival. An active approach to diagnosis and treatment is therefore mandatory.