The occurrence of new primary tumours among postmenopausal patients with primary breast cancer subsequent to adjuvant treatment in Denmark was assessed by linkage to the cancer registry. Following primary surgery, patients in low risk of recurrence (n = 1,828) received no further treatment while patients in high risk randomly received either adjuvant radiotherapy alone (n = 846) or radiotherapy + tamoxifen 30 mg daily for 48 weeks (n = 864). With a median follow-up of 8 years, the incidence of tumours in the contralateral breast was similar among tamoxifen-treated, and non-treated high-risk patients even after adjusting for tumours arising within the first year. The standardized incidence ratio for endometrial cancer was 1.9 (95% confidence interval 0.8-3.9) among tamoxifen treated, the cumulative incidence 1% compared to 0.3% among non-treated patients (p = 0.11). The cumulative risk of non-lymphocytic leukaemia was 0.9% and 0.1% among irradiated and non-irradiated patients respectively (p = 0.4). Prolonged follow-up of tamoxifen-treated patients with regard to new tumours is recommended.