The occurrence, clinical, and epidemiologic characteristics of fatal nosocomial Legionnaires' disease were ascertained by examining a sample of autopsy specimens submitted by hospitals participating in the National Nosocomial Infections Study. Sections of lung tissue from 263 patients were submitted by 40 hospitals in 24 states. Legionnaires' disease was identified in 10 patients (3.8%) from six hospitals in five states. The clinical and epidemiologic characteristics of sporadic fatal nosocomial Legionnaires' disease were similar to those from cases that occurred during epidemics of Legionnaires' diseases. A case-control study showed that cases of Legionnaires' disease were associated with an increased occurrence of Legionnaires' disease during the period June through November, temperatures greater than 39.2 degrees C, and administration of corticosteroids before onset of pneumonia. Legionnaires' disease can occur as a fatal nosocomial pneumonia and has clinical and epidemiologic features that may be helpful in differentiating it from nosocomial pneumonia of other causes.