Four single-arm trials using methotrexate (M), 5-fluorouracil (5FU), and leucovorin (L) were sequentially performed in metastatic measurable colorectal cancer using different dosing and timing schedules for the three drugs. A total of 99 patients were entered into the trials, 92 (49 men and 43 women) of whom were evaluable for response. Although the first trial appeared to have a 36% response rate, the overall response rate seen in the four trials was not higher than historical reports of 5FU alone. We could find no correlation with dose intensity or patient characteristics to account for the apparent good results of the first trial. It is our conclusion that MFL is not an active combination in colorectal cancer. After progressing to the fourth trial, four patients were placed on 5FU alone (supported by a grant from Lederle, Canada). This resulted in one response in a patient who had not responded to the three-drug combination and stable disease in two patients previously progressing. Although there are many reports of this combination working, the possibility is raised that M may be inhibitory to the combination despite the in vitro data.