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A tanker truck carrying anhydrous ammonia (NH3) fell off a freeway, releasing a dense cloud of N H3 gas, killing several people. The driver was dead upon impact. To our knowledge, pulmonary NH3 toxicity in humans has not been studied previously by electron microscopy (EM). Therefore, in two cases, the paraffin-embedded tissue blocks of lung were deparaffinized and reembedded in plastic for 1-μ sections and EM examination. The lung tissue of a third case, the truck driver, was similarly processed as a control. Light-microscopic pulmonary findings in the acute NH3 deaths included denudation of the tracheobronchial epithelium, edema of the lamina propria, and marked alveolar edema, congestion, and hemorrhage. In contrast, in the truck driver's lungs, the bronchial epithelium was intact, and there was no gross odor of NH3. Massive pulmonary hemorrhages in his lungs were attributed to trauma rather than NH3 inhalation. EM examination of the lungs of the truck driver showed no discernible toxic alterations in either the capillary endothelial cells or the Type I or II alveolar epithelial cells, and alveolar and capillary basement membranes were intact. In contrast, EM study of the lungs from two individuals dying acutely of NH3 inhalation showed marked swelling and imbitional edema of Type I alveolar epithelial cells; however, alveolar basement membranes and capillary endothelial cells appeared as usual. These electron-microscopic findings demonstrate the Type I epithelial cell to be the target cell of acute alveolar wall injury in NH3 inhalation.