We characterize the consequences of androgen deprivation therapy on body composition in elderly men. Using a dual energy x-ray absorptiometry instrument, we determined the changes in bone mineral density, bone mineral content, fat body mass and lean body mass in 35 patients with prostate cancer without bone metastases who received luteinizing hormone releasing hormone analogue for 12 months. At baseline conditions 46% of cases were classified as osteopenic and 14% as osteoporotic at the lumbar spine and 40% were osteopenic and 4% osteoporotic at the hip. Androgen deprivation significantly decreased bone mineral density either at the lumbar spine (mean gm./cm. 2 SD 1.00 0.194, 0.986 0.172 and 0.977 0.182 at baseline, and 6 and 12 months, respectively, p <0.002) or the hip (0.929 0.136, 0.926 0.144 and 0.923 0.138, p <0.03). A more than 2% decrease in bone mineral density was found at the lumbar spine in 19 men (54.3%) and at the hip in 15 (42.9%). Bone mineral content paralleled the bone mineral density pattern. Lean body mass decreased (mean gm. SD 50,287 6,656, 49,296 6,554 and 49,327 6,345, p <0.003), whereas fat body mass consistently increased (18,115 6,209, 20,724 6,029 and 21,604 5,923 p <0.001). Serial bone densitometry evaluation during androgen deprivation therapy may allow the detection of patients with prostate cancer at risk for osteoporotic fractures, that is those with osteopenia or osteoporosis at baseline and fast bone loss. The change in body composition may predispose patients to accidental falls, thus increasing the risk of bone fracture.