A total of 65 patients with advanced colorectal or breast cancer were given oral tegafur in the phase I study. Of these, 28 patients had accurately measurable lesions and were entered into the phase II study. Patients with liver metastasis, compromised bone marrow, or a decreased oral intake to less than 50% of their normal intake were given a poor-risk schedule consisting of 0.75 g/m2/day divided into 4 doses/day X 28 day unless early toxicity developed. In the absence of any of the above deficits, patients were given a good-risk schedule of 1.25 g/m2/day X 21 days if no toxicity appeared. The courses were followed by a 2-3-wk rest period depending upon the speed to recovery from all reactions and then repeated. Compared with 5-FU, nausea and vomiting occurred more frequently with tegafur while hematologic toxicity was less common and less severe. The paucity of significant hematologic toxicity permitted courses of this drug to be given safely on an outpatient basis even to the point of slight reaction for optimal dosing. No serious reactions occurred in any of the 65 patients. The clinical results showed partial regressions in 6 of 21 colorectal cancer patients and 3 of 7 breast cancer patients. Six of the 9 responses occurred in patients who had previous 5-FU trials. The average duration of regression in the colorectal cancer patients on oral tegafur was 9 mo with a significantly increased survival of the responders.