We studied the patients admitted to the ICU at a Danish university hospital during 1 year with respect to nosocomial pneumonia (NP). Among 242 patients, who stayed more than 48 h, 23 (10%) developed NP. Patients with NP had significantly higher mortality (43% vs. 19%, p less than 0.05), longer median stay (276 h vs. 99 h, p less than 0.05) and a longer median intubation period (256 h vs. 74 h, p less than 0.05). In the NP group surgical patients were overrepresented as compared to medical patients (74% vs. 45%, p less than 0.05). Thoracotomy, treatment with H-2 blockers and immunosuppression represented significant risk factors. Considering the etiology, Enterobacteriaceae and Pseudomonas aeruginosa constituted 43% of the cases in strong contrast to the low frequency of these pathogens in community-acquired pneumonia. NP in the ICU patient is a resource consuming disease associated with a high mortality (43%), which is related to the frequent severe underlying diseases of these patients.