Twenty-six patients with acute leukaemia and 14 with high-grade lymphoma received cytosine arabinoside (ara-C) at a twice daily dose of 2 g/m2 administered as a 3-h infusion. Thirty-four patients received 12 doses and six electively received four doses only. Complete remission was achieved in six of seven patients with acute myelogenous leukaemia (AML), one of two evaluable patients with blast crisis of chronic myeloid leukaemia and three of eight patients with acute lymphoblastic leukaemia (ALL). Three further patients with ALL had only minimal bone marrow infiltration after one cycle, toxicity precluding administration of a second. Three patients with AML who received four doses only showed no evidence of response. Four of 14 patients with lymphoma who received 12 doses, entered complete remission. Five additional patients died with minimal residual disease whilst severely neutropenic. A complete and a partial response were seen in two patients with immunoblastic and centrocytic lymphoma respectively who received four doses. These results confirm the activity of high-dose ara-C in patients with AML and suggest that it may also be a potentially useful agent in ALL and high-grade lymphoma, especially as the incidence of CNS toxicity is lower than that reported at higher doses.