Risedronate is an orally administered pyridinyl bisphosphonate that is 36 times more potent than pamidronate and 72 times more potent than clodronate. Four randomized, double-blind trials have been carried out in patients with postmenopausal osteoporosis. In 2 of these studies, vertebral fracture incidence was reduced by a daily dose of 5 mg risedronate by up to 65% and 49% relative to placebo after 1 and 3 years, respectively. In these trials, risedronate improved lumbar spine, femoral neck, and femoral trochanter bone mineral density (BMD) at 6 months. In addition, preclinical studies have shown that risedronate is more potent than pamidronate and clodronate in inhibiting adhesion of prostate cancer cells to bone and preventing tumor cell invasion. The incidence of osteoporosis in prostate cancer patients has been well established; therefore, it is advantageous to assess the efficacy of oral bisphosphonate therapy.