We assessed the effects on bone remodeling and histomorphometry after daily subcutaneous injections of teriparatide (n = 39, 20 microg/d) or oral strontium ranelate (SrR, n = 40, 2 g/d) in postmenopausal women with osteoporosis. Evaluable biopsies were obtained from 29 patients in the teriparatide group and 22 in the SrR group after 6 mo of treatment. The mean +/- SD mineralization surfaces as a percent of bone surfaces (MS/BS, %) at the trabecular level were 7.73 +/- 1.48% for teriparatide and 5.25 +/- 1.15% for SrR (p = 0.219) and at the endocortical level were 17.22 +/- 3.06% and 9.70 +/- 2.07%, respectively (p = 0.052). Cortical porosity was 5.40 +/- 0.41% in the teriparatide and 4.14 +/- 0.40% in the SrR group (p = 0.037). Teriparatide induced significant increases from baseline in bone formation and resorption markers, reaching statistical significance for amino-terminal propeptide of type I collagen (PINP) after 1 mo (+57%, p < 0.001). SrR induced small, but statistically significant, reductions from baseline in PINP at 3 (-14%, p = 0.005) and 6 mo (-19%, p < 0.001) and in serum beta-C-terminal telopeptide of type I collagen (beta-CTX) at 1 and 3 mo (-11%, for both, p < 0.05). There were more patients with adverse events after SrR (70%) than teriparatide (41%) treatment (p = 0.013). In conclusion, the changes in biochemical markers of bone formation confirmed bone-forming activity of teriparatide but not of SrR treatment. The effects of SrR on bone remodeling and cell activity were modest, indicating that its effects on fracture reduction may be predominantly mediated through a different mechanism than that observed with anabolic or more potent antiresorptive agents.