A randomized study was performed in 35 centers in the Kinki area of Japan to determine the effectiveness of ftrorafur (FT) plus tamoxifen (TAM) compared with FT monotherapy in postoperative adjuvant therapy for breast cancer. Patients were randomized by the envelope method to receive either FT 600 mg/day or FT 600 mg/day plus TAM 20 mg/day orally for 1 year, starting on day 7 after mastectomy. Between April 1982 and January 1985, 628 patients were assigned to treatment with FT alone and 626 to treatment with FT + TAM. Of these, 571 (90.9%) and 539 (86.1%) patients, respectively, met the eligibility requirements for this study. There were no significant differences in major background factors between the two groups of eligible patients. Five-year survival rates were 91.4% for FT alone and 91.1% for FT+TAM (not significantly different). Five-year disease-free survival rates showed a tendency towards a better prognosis (P = 0.090) in the FT + TAM group, with observed rates of 83.0% for FT alone and 86.7% for FT + TAM. Stratified analysis showed that disease-free survival with FT + TAM is better than with FT alone for patients aged 50 years or more ( P = 0.048) and for patients with from one to three positive nodes (P = 0.064).