The Mayo Clinic and the North Central Cancer Treatment Group (NCCTG) conducted a randomized clinical trial comparing five different combination chemotherapeutic regimens to single-agent 5-fluorouracil (5-FU), given by intravenous bolus technique (500 mg/m2 for 5 days) as a control, in the treatment of advanced colorectal cancer. This report summarizes the results of treatment in 208 patients who were randomized to 5-FU alone or 5-FU with leucovorin in either a high-dose (200 mg/m2) or a low-dose regimen (20 mg/m2) intravenously for 5 days. Both of the 5-FU with leucovorin regimens were associated with improved survival compared with single-agent 5-FU (P less than 0.03). The interval to tumor progression, measurable tumor response rates, and measures of quality of life (performance status, weight gain, and symptomatic relief) were also improved significantly with the addition of leucovorin. There was no therapeutic advantage associated with the use of high-dose compared with low-dose leucovorin. The dose-limiting toxicity of 5-FU/leucovorin was stomatitis. There was one treatment-related fatality (due to sepsis) among the 138 patient receiving 5-FU/leucovorin (0.7%). The most favorable regimen in this trial was 5-FU with low-dose leucovorin, based upon considerations of therapeutic effectiveness, toxicity, and cost. A national intergroup trial is being coordinated by the National Cancer Institute that will test the efficacy of low-dose leucovorin with 5-FU as one approach to adjuvant therapy after a curative surgical resection in selected patients with Dukes' Stage B2 or C colon cancer.