Cyclophosphamide was administered to 42 patients with acute lymphocytic leukemia (ALL) as a daily continuous i.v. infusion at a dose of 400 mg/m2 per day .times. 5 days; the courses of treatment were repeated every 3 wk. Of the 42 patients entered, 21 achieved a complete response, 2 achieved a partial response, 12 failed to respond and 7 died early. The response rate was 69.5% if only patients who received adequate trials are considered; mean duration of response was 18.5 wk and mean survival time was 24.2 wk. Twenty-three patients had relapsed after previous chemotherapy; 19 patients were untreated for advanced high-risk cases of ALL. No difference was found in the response rates, durations of response and survival times between these groups. No significant genitourinary toxicity occurred. Myelosuppression became the dose-limiting toxic effect. Continuous infusion of cyclophosphamide is a clinically effective method of ALL treatment and may have a role in the initial combination regimens for this hematologic malignancy.