This study determined the early natural history of disuse osteoporosis in the ulna and radius. Six women and 2 men (mean age, 48.5 years; range, 35-60 years) having surgery on their wrists or hands had bone mineral density determined by single energy xray absorptiometry at 4 sites of the distal radius and ulna before operation, at cast removal (mean, 4.9 weeks after surgery), and after an average of 4.7 weeks of remobilization and hand therapy. A control group of 4 men and 4 women (mean age, 35.6 years; range, 24-46 years) had bone mineral density measurements of both forearms taken initially and again 5 weeks later. The patients had significant loss in bone mineral density at the ulna and distal sites of the forearm after 4.9 weeks of immobilization. Loss of bone mineral density continued at all 4 sites even after 4.7 weeks of remobilization and hand therapy. Bone mineral density increased significantly at the ultradistal radius of the contralateral forearm (which was not operated on) after 4.9 weeks, but this gain was no longer significant after 4.7 weeks of remobilization of the surgically treated forearm, suggesting that increased activity of the nonimmobilized forearm increased bone mineral density at certain sites. No changes in bone mineral density were seen in the control group. Immobilization of the forearm after hand or wrist surgery significantly decreases bone mass in the distal radius and ulna.