From January 1 1985 to December 31 1990, 874 cases of female primary breast cancer were treated in the Department of Surgery at Beijing Institute for Cancer Research. Of these, 21 patients suffered from contralateral primary breast cancer after surgery. These patients were divided into two groups, 523 patients received adjuvant tamoxifen therapy (20mg daily) for 2 to 5 years as the treated group. There were 351 patients without tamoxifen therapy as the control group. The medium follow-up of the treated and the control group was 7.8 years and 7.0 years, respectively. The incidence of contralateral primary breast cancer in the treated group was 1.5% (8/523), and that in the control group was 3.7% (13/351, P < 0.05). This result suggests that tamoxifen is useful to reduce the risk of contralateral primary breast cancer.