In 14 patients with acute myeloid leukemia (AML) the plasma concentration of cytosine arabinoside (Ara-C) was determined at the start of the first course of treatment at various intervals after a bolus injection. In 10 patients plasma concentration/time data were fitted to a biexponential equation and pharmacokinetic parameters were estimated from the coefficient and exponents of such equations. The plasma half-life (t½) of Ara-C of the first phase varied from 1.2 to 1.9 min (mean 1.6). The t½ of the second phase varied from 8.8 to 18.9 min. All patients were treated with Ara-C alone in a dose of 100 mg/m2 for 10 or 14 days. There was poor treatment response in five patients with second-phase t½ of Ara-C ranging from 6.6 to 10.7 min whereas there was complete remission in nine patients with t½ exceeding 12.7 min. In three patients plasma Ara-C concentrations were measured during constant-rate infusion of different amounts of drug. It appeared that the plateau concentrations were directly proportional to the dose, which indicated that in the therapeutic range no enzyme capacity-limited elimination occurs.