One hundred eighty-one elderly women with stage II breast cancer were prospectively randomized to receive tamoxifen or placebo for 24 months in a double-blind adjuvant trial conducted by the Eastern Cooperative Oncology Group. They were stratified prior to randomization on the basis of the number of positive axillary nodes and the estrogen receptor status of their primary tumor. One hundred seventy were considered eligible and analyzed in this trial. The median age was 71 years with a range from 65 to 84 years. Twenty-one percent of the patients were over the age of 75 and 33% were between 71 and 75 years. Estrogen receptor status was positive in 85% of the patients and unknown in another 12%. Progesterone receptor status was positive in 35%, negative in 16%, and unknown in 49%. With a median follow-up of 55 months, the overall percent disease free at 4 years is 73 for the tamoxifen arm and 52 for the placebo arm (P = .003). Significant benefit is seen following tamoxifen in the subsets with 4-10 positive axillary lymph nodes, those who were estrogen receptor positive, or progesterone receptor unknown, and those who had a tumor size less than 3 cm. Most other subsets benefited as well. There were more distant (29 vs. 13) and bone only (15 vs. 3) sites of first recurrence in the placebo arm, whereas locoregional recurrences were similar (8 each). The recorded toxicity was similar, except for more hot flashes observed among women in the tamoxifen arm.(ABSTRACT TRUNCATED AT 250 WORDS)