The role of antiestrogen treatment in high risk postmenopausal patients with primary breast cancer is currently evaluated in a nationwide, prospective randomized trial conducted by the Danish Breast Cancer Cooperative Group. The primary treatment is total mastectomy and radiotherapy. As of February 1, 1982, 720 women were randomized to treatment with tamoxifen (30 mg daily for 1 year) and 691 women were randomized to no further therapy. Life-table analysis after 36 months shows a difference in recurrence rates of 9% (p = 0.19) in favor of the tamoxifen-treated patients. The material has been analyzed with respect to established prognostic factors such as age, degree of anaplasia, tumor size, and number of positive nodes. The rates of recurrent disease are lower in all subsets of patients treated with tamoxifen, but are only statistically significant in patients 50-59 years of age or with 4 or more positive lymph nodes. Regardless of treatment, ER negative patients have a 23% higher recurrence rate than ER positive patients after 18 months of analysis (p = 0.0033); this represents an approximate doubling of risk, and is independent of age, degree of anaplasia, tumor size, or lymph node status. With regard to PgR status, there is 11% higher recurrence rate in the PgR negative than in the PgR positive patients (p = 0.097).