Eight patients with histological intra-alveolar organization, but no evidence of an infective or other aetiological agent, are reported. They characteristically presented with a short history of severe dyspnoea, cough, malaise, weight loss, bilateral radiographic shadowing and a raised ESR. There was a dramatic response clinically and radiologically to prednisolone but relapse occurred quickly as the dose was reduced. Control was re-established with an increased dose of prednisolone. In order to avoid confusion with post-infective organizing pneumonia the term cryptogenic organizing pneumonitis is suggested.