Potential risk factors for developing Haemophilus influenzae nosocomial pneumonia have not been sufficiently studied. We wanted to investigate the incidence and risk factors for the development of Haemophilus influenzae pneumonia in the hospital by means of a multivariate analysis. A total of 468 cases of nosocomial pneumonia were observed during the study period, 317 (68%) of which were aetiologically diagnosed by means of highly reliable methods, and H. influenzae was isolated in 57 of them. Fifty of the 57 episodes of H. influenzae pneumonia occurred in mechanically-ventilated patients. Underlying diseases were: medical in 12 cases, surgical in 15 cases, and traumatological in 22. Variables associated with Haemophilus influenzae nosocomial pneumonia in intubated patients after the univariate analysis were: "period between admission and pneumonia 2-7 days" and "no previous antibiotics". A multivariate analysis demonstrated that the variables "no previous antibiotics" and "coma on admission" were risk factors for H. influenzae pneumonia. In nonintubated patients, no risk factors were found for H. influenzae pneumonia compared with other nosocomial pneumonia. We conclude that H. influenzae was involved in 57 out of 317 (18%) of nosocomial pneumonia registered in our institution, and the majority of patients (50 out of 57) were mechanically-ventilated. In this particular subgroup, coma of the patient on admission to hospital and absence of antibiotic treatment prior to developing pneumonia constitute two definite risk factors for developing H. influenzae nosocomial pneumonia.