AbstractSixteen patients with metastatic carcinoma of the colon were treated with a regimen of leucovorin 200 mg/m2, given as a 10-min infusion followed by a median dose of 833 mg/m2(range 500–1000 mg/m2) 5-fluorouracil every two weeks. For the 16 patients with proven metastatic disease, two-year survival exceeds 50%. Responses were: 2 complete; 4 partial; 4 minor; 3 progression; and 3 not evaluable but without progression to date. Toxicities included: 8 (50%) leukopenia; 9 (56%), 1 severe, thrombocytopenia; 9 (56%), 2 severe, diarrhea; 9 (56%), 3 severe, nausea/vomiting; 8 (50%), 1 severe, stomatitis; 7 (44%) conjunctivitis; 6 (38%) alopecia; and 13 (81%), 3 severe, neurotoxicity. Leucovorin appears to exert a dose-dependent beneficial effect on both the response and survival produced by the intermittent high dose 5-fluorouracil schedule. This benefit first appears to increase substantially when the leucovorin dose is increased from 120 to 200 mg/m2. Findings identify a testable candidate regimen for selected good risk patients. Full selection criteria remain to be identified.