A phase II study of oral 5'-deoxy-5-fluorouridine (5'-DFUR) was conducted on a multi-institutional basis. Six hundred and ninety-two cases were entered into this study and evaluated by the extramural review committee. In 437 evaluable cases the response rate was 16.0%, 13 CR and 57 PR. The response rate of gastric cancer was 14.3%, 1 CR and 19 PR out of 140 evaluable cases, that of colorectal cancer was 9.2%, 1 CR and 6 PR out of 76 evaluable cases, and that of breast cancer was 35.9%, 11 CR and 26 PR out of 103 evaluable cases. The response rate was high with a long-lasting efficacy in breast cancer. Although the number of cases was small, PR was observed in 3 cases of head & neck, and in 1 case each of esophagus, gall-bladder and thyroid cancer. The optimal daily dosage was considered to be 800-1,200 mg/body/day. The tumor began to decrease in size within 8 weeks of treatment in most of the responded cases. Side effects were observed in 44.4% of 513 cases. Diarrhea occurred with the highest rate (26.3%), but this was controllable.