Forty-nine adult patients with acute leukemia in relapse, refractory to conventional therapy, were studied. Increasing quantities of i.v. bolus high-dose cytosine arabinoside (cytarabine) were administered using the following schedules: 3 g/m2 every 12 hrs for 4-16 consecutive doses, or 4.5 g/m2 every 12 hrs for 12 consecutive doses. Patients ages ranged 16-76 year (median: 38). Thirty-seven patients had previously received either induction or maintainance therapy with conventional doses of cytarabine. Cerebral or cerebellar dysfunction attributable to cytarabine was observed in eight patients and appeared 6-8 days (mean: 6.6) after the first dose and lasted 3-7 days (mean: 4.7). None of 12 patients receiving up to 24 g/m2 total dose of 48 g/m2 developed reversible neurologic dysfunction. Four of six patients receiving 54 g/m2 developed CNS toxicity (irreversible in two cases), a significantly greater incidence compared to toxicity in patients receiving less than or equal to 48 g/m2 total dose (P less than 0.01). CNS toxicity was dose-related since patients treated for 12 consecutive doses of 4.5 g/m2 had significantly greater CNS toxicity than 12 consecutive doses at 3 g/m2 (P less than 0.04). Systemic cytarabine doses less than 54 g/m2 can be administered with minimal CNS side-effects.