A 12-year-old boy with classic dermatomyositis developed a rapidly progressive pulmonary process that led to his death a month later. Physiologically, there was interference with the oxygenation of the blood even when 100% oxygen was delivered to the lungs under pressure. Microscopic examination of the lungs disclosed diffuse interstitial pulmonary fibrosis. We conclude that these pulmonary changes were a component of the basic disease.