The aim of this study was to evaluate the mechanism underlying the beneficial effect of phosphate combined with calcitonin on trabecular bone mass in postmenopausal osteoporosis. Histomorphometric parameters of trabecular bone formation and resorption were assessed blindly on sections from tetracycline-labeled iliac crest bone biopsies from 44 women with postmenopausal osteoporosis obtained before and after 6 months of treatment with phosphate (n = 9), calcitonin (n = 13), combined therapy (n = 13), or double placebos (n = 9). Treatment with phosphate (1.5 g/day) increased the osteoblastic surface in correlation with the fractional trabecular surface with double tetracycline labeling. The mean wall thickness of the basic structural units increased significantly only in the two groups of patients treated with phosphate. Thus, oral phosphate therapy stimulated bone formation by increasing both the bone-forming surfaces and bone matrix production. The mean interstitial bone thickness, which is inversely related to the depth of resorbing cavities, was increased in the two groups treated with calcitonin (50 IU X 5 days every third week), indicating that calcitonin therapy partially inhibited the resorbing activity of osteoclasts. The combination of calcitonin and phosphate produced a reduction in bone resorption associated with a stimulation of bone matrix production. This effect resulted in a 22.1% increase in the thickness of the trabeculae and a 31.1% increase in trabecular bone volume. The data show that calcitonin combined with phosphate increased the trabecular bone volume in postmenopausal osteoporosis through reduction of bone resorption associated with stimulation of bone formation along the trabecular bone surface.