A patient with type IV Ehlers-Danlos syndrome developed recurrent pneumothoraces. At operation for poudrage of the pleura, an apical bulla was resected. Biochemical analysis of the lung tissue revealed increased solubilization of collagen by acetic acid extraction and by pepsin proteolysis. The relative proportion of type III collagen in the pepsin-solubilized collagen fraction was markedly decreased. Fibroblasts cultured from the patient's lung produced less type III procollagen relative to type I procollagen than control lung fibroblasts. Electron microscopic examination of the lung tissue showed dilated endoplasmic reticulum of the fibroblasts, and collagen fibers were normal. These observations suggested that the structural abnormalities in the lung that led to pneumothorax in this patient were due to deficiency of type III collagen.