Exogenous leucovorin is a source of reduced folate which enhances the inhibition of thymidylate synthase that results from 5-fluorouracil (5-FU) administration. Extracellular reduced folate concentrations of 1 microM have been reported to yield maximal enzyme inhibition in several cell lines treated with 5-FU in vitro. Clinical studies indicate that low doses of leucovorin have equivalent efficacy to higher doses in successfully modulating 5-FU in the treatment of colorectal cancer. Based on pharmacokinetics at higher doses, steady-state total plasma reduced folate concentrations of 1 microM would be expected from the administration of leucovorin 50 mg/m2 by 24 h infusion. This dose was admixed with 5-FU 2300 mg/m2 and administered by 24 h-infusion weekly to 38 patients with advanced colorectal cancer, of whom 32 are evaluable for response. Disease sites included liver (33 patients), lung (12 patients), and bone (4 patients). Toxicity was mild to moderate, except for grade 3 diarrhea in 5 patients, and chest pain in 2 patients. Among the 32 evaluable patients, there were 14 partial remissions for a total response rate of 44% (95% confidence interval 27-61%). The median duration of response was seven (range 1 to 20+) months, and median duration of survival 16 months. These results support the use of low doses of leucovorin to modulate weekly infusional 5-FU in colorectal cancer, and provide a basis for the integration of this regimen with other modulators of 5-FU.