In order to study preliminary the safety of 5'-DFUR treatment as postoperative adjuvant therapy, intermittent and continuous treatment regimens were administered to patients undergoing curative resection of carcinomas of the stomach and the colorectum. Two treatment schedules were employed: 5'-DFUR was either given continuously in a daily oral dose of 600 mg/patient (continuous group) or for 2 weeks in a daily oral dose of 1,200 mg/patient followed by 2 weeks of no treatment (intermittent group). Twenty-one stomach cancer patients and 34 colorectal cancer patients were registered in the study. The rates of adverse drug reactions in the patients who completed treatment were 20.0% (2/10) in the continuous group and 50.0% (4/8) in the intermittent group of gastric cancer patients, and 16.6% (2/12) in the continuous group and 17.6% (3/17) in the intermittent group of colorectal cancer patients. The main adverse drug reactions were gastrointestinal symptoms. The incidence of diarrhea, a problematic side effect of 5'-DFUR, was 4.5% (1/22) in the continuous group and 12.0% (3/25) in the intermittent group. There were no statistically significant differences between the continuous group and the intermittent group in regard to the incidence of adverse drug reactions and survival rate. In addition, as there were no serious adverse drug reactions, both treatment regimens were demonstrated to be highly safe when administered as postoperative adjuvant therapy.