The tyrosine kinase inhibitor, imatinib mesylate (Gleevec, Novartis Pharmaceuticals Corp, East Hanover, NJ) (formerly STI571) showed significant antileukemic activity with minimal toxicity in preclinical studies. Based on these data, a phase I clinical trial was conducted in patients with chronic myeloid leukemia (CML) who failed other treatment options. Once therapeutic doses were attained, 53 of 54 patients (98%) in the chronic phase achieved hematologic remissions. With prolonged therapy of 2 to 5 months duration, a growing percentage of patients achieved cytogenetic responses. Imatinib mesylate also has activity as a single agent in CML blast crisis and in patients with Ph(+) acute lymphocytic leukemia (ALL). Although responses tend not to be durable, 20% of patients with myeloid blast crisis are in continuous remission for periods up to 1 year. Ongoing clinical studies are directed at optimizing the use of imatinib mesylate.