The Stockholm Breast Cancer Study Group conducted a randomized trial during 1976 through 1990 comparing adjuvant tamoxifen versus control. A total of 2,738 postmenopausal women with invasive, early stage disease were randomised between tamoxifen for 2 or 5 years, or no adjuvant endocrine therapy. Among high-risk patients the treatment was given against a background of either postoperative, locoregional radiation or CMF-type chemotherapy. The median follow up was 18 years (range 11-25 years). There was a statistically significant (p =0.001) interaction between ER status and tamoxifen with no treatment benefit among receptor negatives. PgR-status had little additional predictive value. Among ER-positive patients tamoxifen reduced locoregional recurrences by 48%, contralateral breast cancers by 54%, distant metastases by 28%, and all events by 24% (p <0.001). On the other hand, there was a substantial increase of endometrial cancer associated with tamoxifen. There was no effect of tamoxifen on intercurrent mortality whereas breast cancer deaths were reduced by 31% (p <0.001) and overall mortality by 15% (p =0.01). Tamoxifen produced long-term benefits among estrogen receptor positive patients in terms of breast cancer-related events, but also an increased incidence of endometrial cancer. Despite long-term follow-up we observed no benefit with tamoxifen in terms of cardiovascular mortality.