The influence of drug interval on the effect of methotrexate and fluorouracil in the treatment of advanced colorectal cancer The importance of the interval between methotrexate (MTX) and fluorouracil (5-FU) was studied in 168 patients with previously untreated, measurable, advanced colorectal cancer. They were randomized to receive MTX 200 mg/m2, followed by 5-FU 600 mg/m2 either 24 hours (arm A) or 1 hour (arm B) after MTX. All patients received leucovorin (LV) 24 hours after MTX, 10 mg/m2 orally every 6 hours for six doses. The regimen was repeated every 2 weeks, with 5-FU escalation as tolerated. Arm A was significantly better than arm B with respect to overall response rate (29% v 14.5%, P = .026), time to progression (TTP; median, 9.9 months v 5.9 months, P = .009), and survival (median, 15.3 months v 11.4 months, P = .003). Significant differences between arms were not found in response rate, median TTP, or median survival for the subgroup of patients with rectal primaries who comprised 20% of the patients in each arm. Significant factors prognostic for survival were performance status and number of metastases, as well as treatment. Age did not influence survival. Toxicity was similar in both arms and was primarily gastrointestinal. More mucositis was seen in arm A. There were four toxic deaths secondary to neutropenia and infection (one from arm A and three from arm B) and three other deaths (two from arm A and one from arm B) that were possibly drug-related. The combination of MTX with LV rescue and 5-FU is an active regimen in advanced colorectal cancer; its efficacy is increased in colon, but not rectal cancer, when the interval between MTX and 5-FU is long (24 hours) rather than short (1 hour). 