Patients (96) with stage III and IV chronic lymphocytic leukemia (CLL) were randomized into 1 of 3 treatment schedules. Prednisone was common to all 3 schedules and was given daily in an initial dosage of 0.8 mg/kg for the first 14 days, with successive halving of the daily dose on days 15 and 29 for a total 6-wk course. Prednisone was then given once a month at 0.8 mg/kg once a day for each of 7 consecutive days. Schedule I was prednisone plus chlorambucil (CLB) given as a once-a-month dose of 0.4-0.8 mg/kg; schedule II was both drugs, but the CLB was given as a daily dose of 0.08 mg/kg; schedule III was prednisone alone. Complete and partial remission (CR + PR) was 47% for schedule I, 38% for schedule II and 11% for schedule III. Patients who responded (CR + PR) in each of the treatment schedules survived longer than nonresponders. Complete remission was obtained in both CLB treatment schedules, but not with the prednisone alone regimen. Although overall survival was best in the intermittent CLB arm, there was no significant difference in survival time between the 3 treatment schedules. Toxicity was minimal in all 3 regimens. Augmentation of the intermittent monthly CLB, even to 1.5 and 2.0 mg/kg, was tolerated without undue marrow toxicity. About 22% of these patients had diabetes mellitus at the time of entry in the study or manifested hyperglycemia during the course of treatment and observation.