Prostate cancer is the most common visceral malignancy and second leading cause of cancer death in men. While androgen ablation therapy is the cornerstone of treatment for more advanced stage disease, recent studies suggest the advantage of introducing androgen deprivation much earlier. Because androgens are essential in maintaining skeletal integrity in men, androgen deprivation therapy constitutes a major risk factor for male osteoporosis. We have previously demonstrated that men on chronic androgen deprivation therapy have up to 20% loss of bone. Our hypotheses are that: 1) chronically increased bone resorption induced by long term androgen deprivation therapy in men with prostate cancer can be reversed with once weekly bisphosphonate; 2) the improvement in bone mass with bisphosphonate therapy will be reflected by changes in biochemical markers of bone turnover and will allow us to predict who will respond to therapy; and 3) following termination of bisphosphonate therapy, bone mass will be maintained despite the absence of antiresorptive therapy. To address these hypotheses, we will enroll 84 men with stage D0 prostate cancer who have been on chronic androgen deprivation therapy in a two year, double blind, placebo controlled, randomized, modified crossover clinical design. During the first year, subjects will be randomized to bisphosphonate therapy or placebo. During the second year, all subjects who were on placebo will receive active treatment and all subjects who were on active treatment will be randomly assigned to continue therapy or change to placebo. To evaluate the effect of bisphosphonate on preventing bone loss, we will assess bone mass of the spine, total hip, total body, and forearm by dual-energy X-ray absorptiometry. For hypothesis 2, we will assess markers of bone resorption and formation to determine if early changes in markers are associated with long term changes in bone mass. For hypothesis 3, we will continue to follow bone mass and biochemical markers of bone turnover between months 12 and 24 to examine rates of change when antiresorptive therapy is terminated. Few data are available on the prevention of bone loss in men on androgen deprivation therapy. This study will examine a preventive strategy, the potential mechanism of bone loss, the ability of biochemical markers to predict bone mass, and skeletal outcomes when antiresorptive therapy is withdrawn.