The aims of this study were to investigate the frequency of bronchopneumonia diagnosed by histological criteria among autopsied intensive care unit (ICU) patients and to compare these with rates of pneumonia diagnosed by conventional clinical methods. The study material comprised 141 autopsied ICU patients from 7 ICUs in university hospitals in Copenhagen from a 1-y period. A total of 20 lung tissue specimens were sampled from each patient and the histopathological diagnoses were classified as no, mild, moderate or severe bronchopneumonia. Inter-observer variation was calculated using kappa statistics. Demographic data and diagnoses of pneumonia were registered from the patient files. Twenty-six percent of the patients had pneumonia diagnosed whilst in the ICU. Histological evidence of pneumonia, found for every second patient, was regarded as the gold standard. Diagnosis of pneumonia in the ICU had a sensitivity of 29% and if diagnoses of pneumonia during the month before ICU-admission were included, a sensitivity of 60% was found. Specificity for pneumonia diagnosed in the ICU was 77%. The percentage of all ICU-patients with pneumonia was calculated to be between 36% and 56%, depending on the extent of excess mortality attributable to pneumonia. Pulmonary segments with histologically diagnosed pneumonic lesions were distributed diffusely, although the upper segments tended to be affected less. Nearly all patients had other histopathological findings than bronchopneumonia. The reliability coefficient among the 6 pathologists was found to be moderately good (kappa = 0.45).