Treatment with corticosteroids can produce osteoporosis. It is generally held that bone loss occurs when steroids are administered daily, but recent findings indicate that bone may also be lost on alternate day therapy. Cortical and trabecular bone, which may be affected differently, can be assessed independently, by quantitative computed tomography. This technique has been applied to the appendicular skeleton in following 20 patients with bronchial asthma during one year of chronic alternate day corticosteroid therapy. The trabecular bone loss was considerable; prednisone 25 mg on alternate days caused an average reduction in trabecular bone of 3.5% over one year. Bone loss was dose- and age-dependent. Young patients on 50 mg/2 days lost up to 17% trabecular bone in one year. Cortical bone was not significantly affected over the same period.