The possibility of increased risk for osteoporosis in breast cancer patients treated with tamoxifen was investigated. 26 patients aged 41-65 years without skeletal metastases were studied. All patients were treated with 20 mg/d tamoxifen for a mean time of 22 months. The data obtained by in vivo neutron activation analysis of the phosphorus content in hands, were supplemented with data obtained by single photon absorptiometry in the forearm and radiographic morphometry. Comparison of the data with that of age and sex matched normal controls showed that breast cancer patients treated with tamoxifen are not prone to osteoporosis.