Cyclophosphamide was administered to 42 patients with acute lymphocytic leukemia (ALL) as a daily continuous iv infusion at a dose of 400 mg/m2/day x 5 days; the courses of treatment were repeated every 3 weeks. Of the 42 patients entered, 21 achieved a complete response, two achieved a partial response, 12 failed to respond, and seven were considered to have early deaths. The response rate was 69.5% if only patients who received adequate trials are considered; mean duration of response was 18.5 weeks and mean survival time was 24.2 weeks. Twenty-three patients had relapsed after previous chemotherapy, and 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.