In a clinical phase-II study fludarabine phosphate was given to 20 patients with advanced chronic lymphocytic leukemia who had failed on prior conventional therapy. Fludarabine was administered at a dose of 25 mg/m2/d for 5 days. Treatment cycles were repeated every 4 weeks until maximal response, followed by two cycles for consolidation. Four of the 20 patients achieved complete remission and seven patients partial remission, resulting in an overall response rate of 55% (11/20). Fludarabine therapy was well tolerated, with mild myelosuppression and secondary infections comprising the predominant side effects. These data warrant further confirmation and a randomized comparison of fludarabine with established regimens, which is currently underway.