We performed a 2-year microbiological study of specimens of sinus fluid recovered from patients with suspected nosocomial sinusitis in six intensive care units. Over that period, 103 cases of sinusitis were documented by positive culture. The number of cases varied widely from one unit to another and was particularly high in the neurosurgical unit. Cultures were often polymicrobial. The main pathogens were Staphylococcus aureus, Pseudomonas aeruginosa, Acinetobacter baumannii and Enterobacteriaceae. Streptococcus pneumoniae and Haemophilus influenzae were far less frequently isolated. Anaerobes and yeasts were often associated with aerobic bacteria. The pathogen involved was also recovered from bronchopulmonary samples in 50 patients and from blood cultures in seven patients.