Two hundred and sixty patients with malignant disease were treated, first with an intramuscular injection of 15 mg of calcium (ca.) leucovorin followed in 16 hours by a continuous infusion of 1000 mg/M(2) of 5-fluorouracil and 15 mg of ca, leucovorin; both given over 24 hours for 5 days. Courses were repeated, toxicity permitting, every 21 days. Severe stomatitis and marrow suppression occurred in every patient but were tolerable and rapidly reversible. Severe diarrhea occurred in only 12 patients. No patient died as a result of therapy. In patients with metastatic carcinoma of the large bowel who had not been treated in the past with a fluorinated pyrimidine, 39% of patients had a partial or complete remission, with a mean duration of response of 16.5 months. In those with carcinoma of the breast who had not received prior non-hormonal therapy, 39% had a partial or complete remission. It is our impression that the response rate and the duration of response in both these neoplasms are substantially better than that seen with fluorouracil alone.