The effect of clodronate on bone mineral density (BMD) was studied in 121 post-menopausal breast cancer women without skeletal metastases. In addition, two antioestrogens, tamoxifen and toremifene, were compared in their action on bone mineral density. Patients were randomized to have an adjuvant antioestrogen treatment either 20 mg of tamoxifen or 60 mg of toremifene daily for 3 years. In addition all patients were randomized to have 1600 mg of oral clodronate daily or to act as control subjects. BMD of the lumbar spine and femoral neck were measured by dual-energy radiographic absorptiometry before therapy and at 1 and 2 years. At 2 years, clodronate with antioestrogens markedly increased BMD in the lumbar spine and femoral neck by 2.9% and 3.7% (P = 0.001 and 0.006 respectively). There were no significant changes in BMD in the patients given antioestrogens only. No significant differences were found between tamoxifen and toremifene on bone mineral density. Clodronate with antioestrogens significantly increased bone mass in the lumbar spine and femoral neck. Both antioestrogens, tamoxifen and toremifene, similarly prevented bone loss in the lumbar spine and femoral neck.