To characterize the pulmonary lesions caused by Serratia marcescens, the authors reviewed all autopsy-culture-proven cases of S. marcescens pneumonia occurring at their hospital between 1968 and mid-1980. In 16, S. marcescens was the only organism cultured from the lungs during life or at autopsy. This report describes primarily these pure infections. Two histopathologic reactions were seen. Nine non-neutropenic patients had acute, hemorrhagic bronchopneumonia, seven with microabscesses and two with larger cavities. In seven, distinctive vasculitis was apparent in vessels larger than 75 microns in diameter; intramural gram-negative rods were identified in two. Seven immunosuppressed patients had diffuse neutropenic pneumonitis resembling diffuse alveolar damage, with extensive intra-alveolar fibrinous exudates and pulmonary hemorrhage. In two patients, bacteria without cellular reaction were present. In patients with prolonged infections, focal areas of intra-alveolar organization and bronchiolitis obliterans accompanied both patterns. Since the incidence of nosocomial S. marcescens infection is increasing and since pneumonia caused by this organism is recognizable histologically, autopsy cultures positive for S. marcescens should not be disregarded.