Bacteriology of acute maxillary sinusitis was studied in 569 patients in 16 centers of 6 countries located throughout Europe during 1992-1994 by ENT specialists. Patients with symptoms of acute sinusitis lasting less than 3 weeks with ongoing purulent nasal discharge were included. Diagnosis was verified by sinus x-ray or ultrasonography and a positive aspiration finding in maxillary sinus puncture. One or more pathogens were isolated from the maxillary sinus aspirates of 375 (66%) patients. Fifty-six percent of patients harboured 1 pathogen and 10% multiple pathogenic organisms, respectively. Haemophilus influenzae was the most common pathogen isolated (148 isolates), occurring as a single pathogen in 14% of the patients. The occurrence of H. influenzae was highest in Finnish military hospital patients (43-48%), as compared with the non-military Finnish patients (9-11%) or to patients from other European centers (mean 13%). H. influenzae was more frequently beta-lactamase positive in other European centers (22%) than in Finnish centers (7%). Streptococcus pneumoniae was the most common pathogen isolated in other European centers (20%) but second most common in Finnish centers (13%). Moraxella catarrhalis occurred at quite similar frequency among Finnish centers (9-14%), but clearly less often in other centers (mean 4%). S. aureus, which in acute maxillary sinusitis is regarded as a contaminant from the nasal cavity, was more prevalent in other European centers (12%) than in Finnish centers (4%). In patients with acute maxillary sinusitis reliable bacteriological samples should be taken by antral aspiration directly from the diseased sinus.