To demonstrate the clinical benefit of 2.5 mg daily risedronate in the treatment of involutional osteoporosis, the effect of risedronate on bone mineral density (BMD) of the lumbar spine was compared with that of etidronate, selected as a representative of the bisphosphonates currently marketed in Japan. In this multicenter, randomized, double-masked, active (etidronate) controlled comparative study, a total of 235 Japanese patients with involutional osteoporosis were randomized to receive either treatment with 2.5 mg/day of risedronate for 48 weeks or intermittent treatment with etidronate (4 cycles of 2 weeks of treatment with 200 mg/day followed by 10-week medication-free periods). All patients received 200 mg of calcium supplement daily in the form of the calcium lactate. Bone mineral density of the lumbar spine (L2-L4 BMD) was determined at 12, 24, 36 and 48 weeks by dual-energy X-ray absorptiometry. The primary endpoint was the percent change in L2-L4 BMD from baseline to the time of final evaluation. Changes in biochemical markers of bone turnover and safety profiles were also compared. A significant increase in L2-L4 BMD was observed at 12 weeks after initiation of therapy in both the risedronate (2.8%) and etidronate (1.8%) groups. The increase in L2-L4 BMD at the time of final evaluation in the risedronate group (4.9%) was significantly greater ( p = 0.002) than that in the etidronate group (3.1%). The changes in bone resorption markers (urinary total deoxypyridinoline and N-terminal telopeptide of type I collagen) from baseline to 48 weeks were -37.6% and -41.3% for risedronate and -22.5% and -26.6% for etidronate, respectively. New vertebral fractures or deterioration of existing fractures were observed in 2.8% (3/106) of the patients in the etidronate group, while no such cases (0/101) were observed in the risedronate group. No significant difference in the incidence of adverse events was found between two treatments. Daily oral risedronate (2.5 mg) exhibited efficacy superior to that of intermittent cyclical etidronate (200 mg) in increasing L2-L4 BMD, and was well tolerated by Japanese patients with involutional osteoporosis.