At concentrations >0.1 mM, hydroxyurea (HU) enhances the accumulation of cytosine arabinoside (ara-C) in leukemia cells in vitro. This study of children with refractory acute leukemia was designed to take advantage of this biochemical modulation. A fixed dose of HU and an escalating dose of ara-C were used. Oral HU (1200 mg/m2) was followed 2 hours later by ara-C (250-3100 mg/m2) intravenously in 15 minutes. The combination was given on days 1, 2, 3 and 8, 9, 10. Thirty-three children [26 acute lymphocytic leukemia (ALL), 7 acute nonlymphocytic leukemia] were treated; 29 received at least 1 full course. All patients developed grade 4 cytopenias. Other grade 3 to 4 toxicities included hyperbilirubinemia (2), elevated transaminases (3), transient gait disturbance (1), stomatitis (3), typhlitis (1), nausea/vomiting (9), and marrow aplasia >4 weeks (1). Three patients had intracranial bleeds while thrombocytopenic. Only liver toxicities and nausea/vomiting exhibited any dosage effect. The maximum tolerated dose of ara-C was 2400 mg/m2. There were 6 complete responses (5 ALL), 5 partial responses (3 ALL), and 19 patients with no response or progressive disease. There was no dosage effect for response with 2 complete responses occurring at the lowest ara-C level. Responses were transient (1 to 3 mo). Twenty of twenty-six patients achieved a peak serum HU level >0.5 mM by 2 hours after the HU dose. The mean level at 2 hours was 0.57 mM (range: 0.21 to 0.99 mM). This combination of HU and ara-C is tolerable and has efficacy in refractory leukemias. Responses at the lowest ara-C dose level suggests synergism.