Between May 1978 and March 1982, 179 postmenopausal women with operable breast cancer were randomized to receive either adjuvant tamoxifen, 40 mg daily for three years (TAM group), or no further treatment (controls). The difference in five-year survival rates (61% in the control group, 72% in the TAM group) was not statistically significant. However, there was a significant improvement in disease-free survival in the TAM group (61%) relative to the controls (44%) (p = 0.008). In estrogen receptor positive patients, tamoxifen improved both the disease-free rate (47% controls, 80% with tamoxifen) and the survival rate (63% to 83%). Similar results were observed in progesterone receptor positive patients. In patients that were estrogen receptor negative, tamoxifen modified neither the survival rate nor the disease-free interval.