Based on estrogen receptor (ER) status and menopausal status, operable breast cancer (International Union Against Cancer [UICC] Stage I, II, and III) patients were randomized for adjuvant endocrine therapy, chemotherapy, and chemoendocrine therapy, and the effects on the disease-free survival (DFS) and overall survival (OS) were compared. Adjuvant endocrine therapy was composed of tamoxifen (TAM) 20 mg/day orally for 2 years in postmenopausal patients. In premenopausal patients, oophorectomy (OVEX) was done before TAM administration. In the chemotherapy arm, the patients were given 0.06 mg/kg of body weight of mitomycin C (MMC) intravenously (IV) and then an oral administration of cyclophosphamide (CPA) 100 mg/body orally in an administration of a 3-month period and a 3-month intermission. This 6-month schedule was repeated four times in 2 years. As the chemoendocrine therapy arm, TAM with MMC + CPA chemotherapy was added. The patients were randomized according to ER and menopausal status. Estrogen receptor-positive (ER+) cancer patients were randomized to three arms: TAM +/- OVEX, MMC + CPA, or MMC + CPA + TAM. For estrogen receptor-negative (ER-) patients, there were two arms: MMC + CPA, or MMC + TAM. The study started in September 1978, and 692 patients entered until the end of 1984 were evaluated. The median follow-up was about 46 months. Totally, a 9.8% rate (68/692) of recurrence was noted, a 7.5% rate (52/692) of mortality. There were no significant differences in DFS or OS among the treatment arms in ER+ or ER- patients. There was significant differences in adverse effects such as bone marrow suppression, gastrointestinal disturbances, cystitis, hair loss between endocrine therapy and chemotherapy or chemoendocrine therapy groups. In this preliminary study, it was concluded that because of less adverse effects of endocrine therapy, it seems rational to select the operable breast cancer patients by the presence or absence of ER, namely, endocrine therapy for ER+ and chemotherapy for ER- cancer patients.