Electron microscopy in an infant of 4 months with pulmonary alveolar lipoproteinosis showed filling of the alveoli with osmiophilic lamellar bodies. Similar structures were present in the cytoplasm of type I alveolar epithelial cells and to a lesser extent in the capillary endothelium and interstitium. These changes represent widespread degenerative processes in the lung caused by an unidentified cytotoxic agent. In this patient the disease is comparable to the drug-induced cytotoxic animal model and differs from the dust-induced hypersecretory animal model.