The DBCG 77C trial compared one year of tamoxifen in postmenopausal, steroid-receptor unknown, high-risk breast cancer patients to no adjuvant systemic therapy. After a potential follow-up of 30 years we report overall efficacy of the study and results according to subtypes subsequently assessed by immunohistochemistry and fluorescent in situ hybridisation (FISH). Methods Between 1977 and 1982, 1716 postmenopausal patients with tumours larger than 5 cm or positive axillary nodes were randomly assigned to no systemic therapy or tamoxifen 30 mg daily for one year. Archival tumour tissue from 1515 patients was analysed and the hormone receptor positive (estrogen receptor (ER) and/or progesterone receptor (PR)) cancers were defined as luminal A if Ki67 low and HER2-negative; as luminal B if Ki67 high or HER2-positive; and otherwise as non-luminal-HER2 positive or triple negative. Findings In the intent-to-treat (ITT) population one year of tamoxifen improved the disease-free-survival (DFS) (hazard ratio (HR) = 0.87; 95% confidence interval (CI) 0.77-0.98), the Breast Cancer Recurrence Rate (BCRR) (HR = 0.79; 0.69-0.90) and reduced the breast-cancer-specific-mortality (BCM) (HR = 0.83; 0.73-0.93). BCRR were improved significantly by tamoxifen in luminal A (HR = 0.66; 0.53-0.84) and luminal B/HER2- (HR = 0.54; 0.39-0.74) but not in the other subsets, and with similar results for BCM with 30 years follow-up. Interpretation One year of treatment with tamoxifen significantly improves BCRR and BCM in postmenopausal patients with ER positive breast cancers. The relative benefit from tamoxifen was not significantly different in luminal A and B subtypes. Funding The Danish Breast Cancer Cooperative Group (DBCG) prepared the original protocol (DBCG 77C) and was the sponsor of the study. Funding was not provided to the participating departments. The biomarker study was supported by grants from the Clinical Institute, Odense University.