Background: Studies have shown the importance of apoptosis in vascular injury in vitro. We postulated that apoptosis of the endothelium contributes to vascular injury in vivo and may be involved in acute lung injury. Methods: To test this hypothesis, we investigated the incidence of endothelial cell apoptosis in acute lung injury induced in mice by the administration of lipopolysaccharide (LPS). Male ICR mice were administered LPS (20 mg/kg body weight) intravenously and sacrificed at specified times thereafter. Results: Histologic findings were consistent with acute lung injury which increased with time from 3 to 48 h after injection. Electrophoretic analysis of DNA that was extracted from lung tissue and 3'–end–labeled with digoxigenin demonstrated a fragmentation of DNA starting at 6 h. In situ terminal deoxynucleotidyl transferase–mediated dUTP biotin nick end–labeling (TUNEL) demonstrated DNA strand breaks in the endothelial cells. TUNEL also revealed DNA strand breaks in bronchial and alveolar epithelial cells as well as inflammatory cells in the interstitium. These TUNEL–positive cells appeared 6 h after injection. Electron–microscopic examination of the endothelium strongly suggested the morphological characteristics of apoptosis. Conclusion: Apoptosis was induced by LPS administration in endothelial cells in vivo. A role for such apoptosis is suggested in acute lung injury.