Male osteoporosis is increasingly recognized as a major public health issue. This multinational, 2-yr, randomized, double-blind, placebo-controlled study was conducted to determine the efficacy and safety of 35 mg once-a-week risedronate in men with osteoporosis. Patients had to be men >or=30 yr old, with lumbar spine T-score <or= -2.5 and femoral neck T-score <or= -1 SD or lumbar spine T-score <or= -1 and femoral neck T-score <or= -2 SD (based on young normal men). Patients were randomized 2:1 to risedronate 35 mg once a week or placebo for 2 yr; all patients took 1000 mg elemental calcium and 400-500 IU vitamin D daily. Lumbar spine BMD at month 24 using last observation carried forward was the primary endpoint. Other endpoints included lumbar spine BMD at time points other than month 24, proximal femur BMD, bone turnover markers (BTMs), new vertebral fractures, clinical fractures, and adverse event (AE) assessment. There were 284 men enrolled in the study. Treatment with risedronate resulted in a significant increase from baseline to endpoint in lumbar spine BMD compared with placebo (4.5%; 95% CI: 3.5%, 5.6%; p < 0.001). Few new vertebral and nonvertebral fractures were reported, with no differences in fracture rates between the two groups. There was a significant (p < 0.01) reduction from baseline in BTMs for the risedronate group compared with placebo at all time points. No apparent differences in the pattern or distribution of AEs including serious and upper gastrointestinal AEs were observed. Risedronate therapy was well tolerated during this 2-yr study and was rapidly effective as indicated by significant BTM decreases at month 3 and BMD increases at month 6 (the earliest time points tested). The effects of risedronate treatment on BMD and BTMs in this study were similar to those previously shown to be associated with fracture risk reductions in women with postmenopausal osteoporosis.