The aim of the present study was to analyze the efficacy of adjuvant tamoxifen (TAM) in postmenopausal patients with high-risk breast cancer. The primary surgical treatment was total mastectomy with axillary sampling. There were 1,650 eligible patients; 829 were randomized to receive postoperative radiotherapy (RT) and 821, to receive RT + TAM (10 mg three times daily for 1 yr). The 2 groups were identical with respect to age, tumor size, number of positive lymph nodes, degree of anaplasia, and estrogen and progesterone receptor content. Overall recurrence-free survival at 6 years was 39% in the RT group, compared to 48% in the RT + TAM group (P = 0.0008), but there was no significant difference in survival (P = 0.14). From retrospective analyses of recurrence-free survival according to prognostic variables, it appears that 4 subgroups of patients benefited from adjuvant TAM: those less than 69 years of age, those with 4 or more positive nodes, those with grade I-II tumors, and those with high estrogen receptor values (greater than 100 fmol/mg cytosol protein).