This paper presents the results of a two-center, double-masked, placebo-controlled, randomized, oral-dose study of risedronate treatment in postmenopausal osteoporosis. Patients had at least one, but no more than four prevalent vertebral fractures at baseline. They received either 2.5 mg continuous risedronate, 2.5 mg cyclic risedronate, or placebo for 2 years. Both risedronate and placebo were formulated as hard gelatin capsules. All women furthermore received a daily calcium supplement of 1 g which was taken separately from the study drug. During the 1-year of follow-up, all women received only a daily calcium supplement of 1 g. A total of 132 patients were enrolled (44 in each treatment group), of which 73% completed the 2-year treatment period and 70% all 3 years. Generally the outcome of the study was negative. Lumbar spine bone mineral density (BMD) increased 1.2% (NS) and 0.8% (NS) and after 2 and 3 years in the group treated with continuous risedronate, 1.7% (NS) and 2.3% (p < 0.05) in the group treated with cyclic risedronate, and 0.6% (NS) and 1.7% (NS) in the placebo group. BMD in the femoral neck increased 2.9% (p < 0.05) and 0.9% (NS) after 2 and 3 years in the group treated with continuous risedronate, 1.3% (NS) and 2.4% (p < 0.01) in the group treated with cyclic risedronate, and 1.3% (NS) and 2.6% (p < 0.01) in the placebo group. The differences between all three groups in spinal and femoral BMD after 2 years were not statistically significant, but reached statistical significance after 3 years (p < 0.01) in the femoral neck. Only minor changes were observed in the measured markers of bone turnover. Both the incidence and rate of new vertebral fractures showed no overall differences between the groups. The distribution of adverse events was similar across treatment groups. None of the serious adverse events were considered causally related to risedronate. The lack of effect shown in the present study may be explained by insufficient dose regimen and/or impaired absorption from the intestinal tract. Further investigations (ongoing phase III trials) are needed to define future dose regimens in order to validate the effect on bone mass, fracture rate and biochemical markers. In these studies another formulation of the drug and other dosing instructions are used.