We describe the clinical features and course of disease in 56 patients (33 men and 23 women) with transient regional osteoporosis. The median age at hospitalization was 48 years. All patients had monoarticular or oligoarticular joint pain that was aggravated by weight bearing. Eighteen also had pain at rest in the involved joints and 6 had nocturnal pain. The joints of the lower extremities were affected in 54 patients; the hip was the most commonly affected joint. There was little evidence of synovitis. Osteoporosis was diagnosed on the basis of characteristic clinical findings and the results of roentgenograms and bone scans. The osteoporosis resolved spontaneously over several months. Treatment consisted of joint protection, initiation of gradual ambulation, and administration of nonsteroidal anti-inflammatory agents. Although its cause is unknown, osteoporosis appears to be a regional rather than a systemic process.