This phase I study of 36 patients was performed to identify the maximal tolerated dose of oral calcium leucovorin (CLV) that could be concomitantly administered with protracted 5-fluorouracil (5-FU) infusion (greater than 30 days). Administration of CLV in very small doses (5 mg p.o. q8h) with 5-FU, 200–300 mg/m2/day, resulted in excessive toxicity requiring treatment interruption in all patients. Subsequent reduction in the dose of 5-FU to 100 mg/ m2/ day with simultaneous administration of CLV, 5 mg p.o. q8h, decreased the toxicity and allowed for protracted administration of the combination. In subsequent patients the dose of oral CLV was increased to 22.5 mg p.o. q8h, which resulted in treatment-limiting toxicity in the majority of patients. Toxicity consisted almost exclusively of mucositis. No mye-losuppression or significant organ toxicity was observed. We conclude that even low doses of oral CLV potentiate the biological effect of infusion 5-FU. If the combination is to be given on a protracted basis, 5-FU must be administered at a much smaller dosage than has been traditionally utilized.