Coadministration of eniluracil with 5-fluorouracil (5-FU) allows the oral absorption of small doses of 5-FU, resulting in therapeutic plasma levels. A phase II clinical trial of this combination using a continuous dosing schedule was carried out in patients with metastatic colorectal cancer. Fifty-three previously untreated patients and 46 patients who had received one prior regimen for metastatic disease were enrolled. Patients received 10 mg/m2 of eniluracil and 1 mg/m2 of 5-FU twice daily for 28 days, with cycles repeated after a 7-day rest until progression of disease or prohibitive toxicity. Seven of 53 previously untreated patients had an objective tumor response (13.2%): 1 complete response and 6 partial responses. The mean duration of response was 6.3 months. Only 1 of the 45 evaluable patients in the previously treated group had a partial response, with no complete responses. The duration of response was 3 months. The median progression-free survival was 3.4 months for the previously untreated group and 2.5 months for the previously treated group. Median overall survival was 11.1 months for the previously untreated group and 9.0 months for the previously treated group. Hematologic toxicity was infrequent, with 3 patients experiencing grade 3 toxicity. Incidence of grade 3/4 toxicity included 11 patients with diarrhea, 5 with nausea, and 4 with vomiting. Other common toxicities included anemia and stomatitis, but they were generally mild. This regimen is well tolerated and shows activity in previously untreated patients with metastatic colorectal cancer that is similar to that observed with other 5-FU-based regimens. This regimen has not shown to be effective in patients who have had prior chemotherapy.