Osteoporosis is not usually considered to be reversible, as it is a consequence of the ageing process. However, an improvement of bone mineral density after successful surgery in Cushing's syndrome has been shown in several reports. The question of reversibility of exogenous corticosteroid-induced osteoporosis is, as vet, unanswered, possibly because of the difficulty in discontinuing steroids after long-term use. We describe six patients, all under 45 yrs of age, with chronic long-standing sarcoidosis, in whom long-term prednisone therapy resulted in 15+/-7% bone loss, as evaluated by quantitative computed tomography. This side-effect appeared fully reversible after prednisone withdrawal. This report of the reversibility of exogenous corticosteroid-induced bone loss needs confirmation in elderly people, where the capacity for recovery of bone mass could be reduced. Such potential for recovery may have implications for the pattern of use of corticosteroids.