Thirty-six patients with advanced colorectal carcinoma who had not received prior chemotherapy and had tumor sites measurable or evaluable for response received methyl-CCNU at a dose of 175 mg/m2 orally on Day 1 plus 5-fluorouracil at a dose of 30 mg/kg/24 hours by continuous iv infusion for 120 hours (Days 1-5). Doses were chosen to approach the maximum which could be administered in combination. The cycle was repeated 6 weeks later. Objective partial responses occurred in six patients (17%), with five responses apparent after the first cycle and one additional response after the second cycle. The response duration ranged from 1.5 to 18+ months (median, 5 months). Dose-limiting toxic effects included mucositis in 18 patients (50%) and dermatitis in nine patients (25%), while leukopenia (less than 4000 cells/mm3) and thrombocytopenia (less than 100,000 platelets/mm3) were observed on at least one occasion during therapy in 52% and 46% of patients, respectively. 5-Fluorouracil administration by infusion avoided overlapping myelosuppression and allowed a higher total dose to be given with methyl-CCNU. However, the response to the combination did not exceed the results anticipated for the use of either drug alone.