The anti-estrogen tamoxifen is the prevalent endocrine treatment in postmenopausal breast cancer patients. However, nothing is known about the long-term effects of the drug on the skeleton as assessed by the occurrence of fractures. We investigated the occurrence of fractures of the femur in patients from a Danish Breast Cancer Cooperative Group (DBCG) trial initiated in 1977 by a linkage of data from the Danish National Registry of Patients with data from the DBCG registry. 1716 postmenopausal women with high-risk breast cancer were randomized to local radiotherapy with or without tamoxifen, 30 mg daily for 1 year. Fifty-one patients in the control group had one femoral fracture and 64 tamoxifen treated patients had one femoral fracture. Eleven patients in the control group had one trochanteric fracture compared to 27 patients in the tamoxifen group (logrank = 5.28. P = 0.022; hazard ratio = 2.12, 95% CL 1.12, 4.01). The results could not be explained by a longer survival in the tamoxifen group nor by bone metastases with pathological fractures. In conclusion, our study suggests that tamoxifen does not seem to offer protection against fractures in old age and may even increase the risk of fractures at particular sites. This hypothesis needs to be disproved or confirmed in other trials.