Open lung biopsies from three patients with Legionnaires' disease were examined by light and transmission electron microscopy. The patients had serious underlying disease. All developed a rapidly progressive pneumonia unresponsive to penicillin, oxacillin, and gentamicin. One patient, who received erythromycin, survived. Light microscopy in all three showed severe acute bronchopneumonia. The Legionnaires' disease bacterium was seen in tissue sections and confirmed by direct immunofluorescence. Transmission electron microscopy showed numerous rod-shaped intracellular organisms that were morphologically similar to other gram-negative bacteria and the Rickettsieae. They were within phagolysosomes, free in the cytoplasm, and rarely within structures resembling dilated rough endoplasmic reticulum. Lung tissue changes included marked detachment and necrosis of alveolar pneumocytes, septal and alveolar exudate with lysis, and prominent endothelial cell swelling and degeneration. Capillary and epithelial basement membranes were consistently intact, suggesting that the tissue changes are potentially capable of reverting to normal structure and function.