Tamoxifen and toremifene are two mostly used antiestrogens in the treatment of breast cancer. To compare their effect on bone in postmenopausal breast cancer patients we measured the urinary output of two bone resorption markers, pyridinoline (Pyr) and deoxypyridinoline (Dpyr) as well as bone density (BMD) in 30 breast cancer patients using either tamoxifen (20 mg/day, n = 15) or toremifene (40 mg/day, n = 15) as adjuvant treatment of stage II breast cancer for 1 year. The urinary output of Pyr and Dpyr were assessed before and after 6 and 12 months of the antiestrogen regimen. Lumbar and femoral BMD were measured by dual energy X-ray absorptiometry (DXA) before and after 12 months of treatment. Both tamoxifen and toremifene were associated with significant decreases in Pyr (mean fall 19.6% and 12.6%, respectively) and Dpyr (mean fall 21.6% and 15.5%, respectively) at 6 months. After 12 months' treatment, Pyr decreased by 30.8% and Dpyr by 21.2% in women using tamoxifen and significantly less in women using toremifene (10.1% and 4.9%, respectively). BMD in the lumbar spine decreased by 1.8% in the toremifene group but increased by 0.4% in the tamoxifen group; in the proximal femur, BMD increased slightly during both tamoxifen and toremifene treatment in all sites measured. Individual changes in Pyr and Dpyr at 6 months showed no significant relation to the change in BMD at 12 months. We conclude that tamoxifen (20 mg/day) and toremifene (40 mg/day) reduce the bone resorption similarly, and this can be detected by falls in urinary output of Pyr and Dpyr at 6 months of treatment.