Pseudomonas pneumonia is one of the fatal opportunistic infections. We examined Pseudomonas aeruginosa in the autopsied lung tissues by the immunostaining (Biotin-Streptoavidin peroxidase complex method). Out of 185 autopsied cases, fifty-three cases had bacterial pneumonia. We performed the immunostaining to these 53 cases, of which 14 cases (26.4%) were diagnosed as pseudomonas pneumonia. All of the 14 cases had severe underlying diseases which were hematologic malignancies in six, solid malignancies in five and so on. Of ten of them pseudomonas pneumonia was considered to be the direct cause of death. P. aeruginosa was observed in alveolar space in all cases, and in bronchial and bronchiolar lumen in many cases. P. aeruginosa was also detected in alveolar and vascular walls. Histopathologically necrotizing vasculitis and alveolar destruction were characteristic, and intra-alveolar hemorrhage, fibrinous exudate, abscess formation, coagulation necrosis and thrombosis were observed in most cases. Necrotizing vasculitis and intra-alveolar hemorrhage were more severe in the cases of leukopenic patients.