The effects of 3 years of oral risedronate treatment on bone quality and remodeling were assessed in women with postmenopausal osteoporosis. Transiliac bone biopsies were obtained at baseline and after treatment with placebo or risedronate 5 mg/day in 55 women (placebo, n = 27; risedronate 5 mg, n = 28); these pairs of samples allowed comparison of treatment effects vs. both baseline values and between treatment groups. A further 15 women (placebo, n = 6; risedronate 5 mg, n = 9) had measurements from a posttreatment biopsy, but not from a baseline biopsy. Samples were examined for qualitative changes (e.g., osteomalacia, peritrabecular fibrosis, and woven bone); no histological abnormalities were found to be associated with treatment. Among women with both baseline and posttreatment biopsies, risedronate-treated women experienced a moderate and expected reduction from baseline in bone turnover, which was reflected in mean decreases in mineralizing surface of 58% and in activation frequency of 47%. Histomorphometrical parameters indicated that bone formation rate decreased significantly from baseline with risedronate treatment, reflecting a decrease in bone turnover; bone mineralization was normal following treatment. Basic multicellular unit (BMU) balance tended to improve in the risedronate-treated women, whereas it tended to worsen in the placebo-treated women, although these changes were not statistically significant. There were no significant changes in structural parameters with treatment. The effects of 3 years of risedronate treatment on bone histology and histomorphometry reflect the antiresorptive mechanism of action, and are consistent with the antifracture efficacy and favorable bone safety profile demonstrated in large clinical trials.