Based on estrogen receptor (ER) and menopausal status, operable breast cancer (UICC stage I, II, III-a) patients were randomized for adjuvant endocrine therapy, chemotherapy, and chemoendocrine therapy, and the effects on the relapse-free survival (RFS) and overall survival (OS) were compared. Tamoxifen (TAM) 20 mg/day was administered orally for 2 years after mastectomy as an adjuvant endocrine therapy in postmenopausal patients. In premenopausal patients, oophorectomy (OVEX) was performed before TAM administration. In the chemotherapy arm (CHEM), patients were given 0.06 mg/kg of body weight of mitomycin C (MMC) intravenously, followed by an oral administration of cyclophosphamide (CPA) 100 mg/day in an administration of a 3-month period and a 3-month intermission. This 6-month schedule was repeated 4 times in 2 years. The chemoendocrine arm (CHEM + TAM) was composed of TAM with MMC + CPA chemotherapy. The patients were randomized according to ER and menopausal status. ER-positive patients were randomized to three arms: OVEX +/- TAM, CHEM, and CHEM + TAM. For ER-negative patients there were two arms: CHEM and CHEM + TAM. 1579 patients entered the trial between September 1978 and December 1991, with median follow-up of 8.2 years. In ER-positive, premenopausal patients, there were no significant differences in RFS or OS among OVEX + TAM, MMC + CPA, TAM + MMC + CPA arms. On the contrary, in ER-positive, postmenopausal patients, the chemoendocrine therapy showed a significantly higher RFS (p = 0.0400) and OS (p = 0.0187) as compared with TAM to chemotherapy alone. There were no significant differences in RFS or OS by addition of TAM on the chemotherapy, in both pre- and post-menopausal ER-negative patients. It was concluded that in ER-positive premenopausal breast cancer, endocrine therapy alone may be equivalent in prolonging RFS and OS to chemotherapy or chemoendocrine therapy, and that ER-positive postmenopausal breast cancer may be better controlled with the combination of TAM and chemotherapy, as compared to TAM or chemotherapy alone. The importance of stratification of operable breast cancer by ER and menopausal status, as well as the direct comparisons of different treatments, were stressed.