1, 25-Dihydroxyvitamin D3 (1, 25-(OH)2D3) has been shown to inhibit breast cancer cell growth both in vitro and in vivo. A major drawback is that high doses of 1, 25-(OH)2D3 are needed which may result in undesirable side effects like the development of hypercalcemia and an increased risk of bone metastases due to the stimulation of bone resorption by 1, 25-(OH)2D3. Several newly developed 1, 25-(OH)2D3 analogs have a reduced calcemic activity, but their direct effects on bone resorption have not yet been examined. Presently, the antiestrogen tamoxifen is the most important endocrine therapy for breast cancer. Recent studies have demonstrated the benefit of the combination tamoxifen and 1, 25-(OH)2D3/analogs for the inhibition of breast cancer cell growth. Besides inhibition of breast cancer growth tamoxifen appeared to have beneficial effects on bone. The purpose of the present study was to investigate the effect of tamoxifen on 1, 25-(OH)2D3- and analogs (EB1089 and KH1060)-stimulated bone resorption in an in vitro model. Bone resorption was stimulated by 1, 25-(OH)2D3 and analogs in a dose-dependent manner with KH1060 and EB1089 being more potent and 1, 25-(OH)2D3. Tamoxifen caused a strong dose-dependent inhibition (70% at 10 microM) of 1, 25-(OH)2D3- and EB1089-stimulated bone resorption. KH1060-stimulated bone resorption was also inhibited by tamoxifen but to a lesser extent (36%). Also the pure antiestrogen ICI164, 384 but not 17 beta-estradiol inhibited 1, 25-(OH)2D3-stimulated bone resorption. Together, this study demonstrates that tamoxifen considerably reduces 1, 25-(OH)2D3/analogs-stimulated bone resorption and therefore may be useful to reduce the risk of bone metastases. This together with the observed beneficial effects on breast cancer cell growth indicates that tamoxifen together with 1, 25-(OH)2D3/analogs is an interesting combination for the treatment of breast cancer. The mechanism of the bone resorption inhibitory action is not yet known but seems to be independent of the estrogen pathway.