Our objective was to investigate the efficacy and tolerability of risedronate in postmenopausal women with low bone mass. Women with a mean lumbar spine T-score of -2 or less (n = 543) received 24 months of placebo or risedronate (2.5 or 5 mg/day). All received calcium (1 g/day). The principal outcome measures were bone mineral density (BMD) at the lumbar spine, femoral neck, and femoral trochanter. At 24 months, lumbar spine BMD increased from baseline by 4% with 5 mg risedronate and 1.4% in the 2.5-mg group, compared with no change with placebo. Efficacy was similar in women who were less than 5 yr and more than 5 yr postmenopausal. At 24 months, risedronate (5 mg) had also increased BMD at the femoral neck and trochanter, whereas BMD decreased in the placebo group. BMD increases were seen at all three sites with risedronate (5 mg) after only 6 months of therapy. Risedronate was well tolerated; upper gastrointestinal adverse events were similar to placebo. We conclude that risedronate (5 mg) increases BMD rapidly and effectively and is well tolerated in postmenopausal women with low bone mass, regardless of time since menopause.