Twenty-nine patients with advanced colorectal carcinoma were entered in this study to evaluate the efficacy and toxicity of a sequential chemotherapeutic schedule with methotrexate (MTX), 200 mg/m2 intravenously (IV) (push injection) and 5-fluorouracil (5-FU), 1,200 mg/m2 in continuous IV infusion, using a 20-hour time interval. All patients received calcium leucovorin (LV), 25 mg, intramuscularly (IM) every six hours for eight doses beginning 24 hours after methotrexate administration. Courses were administered every 15 days. Of the 24 patients evaluable for response, 11 (46%) had major objective regressions (one complete remission [CR] and ten partial remissions [PR]). The survival rate of patients who responded to treatment was 60% at 16 months, whereas patients with no change and those in whom the disease progressed had a median survival of 9 months and 3 months, respectively. The median duration of response has not yet been reached in patients who presented objective tumor regression, and was 7.5 months in those with no change. Significant differences were found between objective regression and no change (P less than .0005) and between no change and tumor progression (P less than .05). All patients were evaluable for toxicity. There were three toxic-related deaths (10%) because of severe myelosuppresion, sepsis, and hemorrhage. These promising results, despite important toxicity, reveal the synergism between the two chemotherapeutic agents and also indicate that the response rate achieved could be a consequence of the 20-hour interval and high dose of 5-FU. Further studies are necessary to determine the optimal time interval and the adequate 5-FU dose.