The aim of this work was to study the bacterial flora of purulent secretions during chronic rhinosinusitis. We studied a total of 533 patients divided into two groups. The control population consisted of 139 adults (> 16 years) of both sexes seen in the community or hospitalized for less than 72 hours for non-rhinological conditions. The rhinosinusitis group consisted of 394 patients referred to the ENT clinic with chronic rhinosinusitis. All the patients with rhinosinusitis had had a post-nasal discharge for at least three months, associated with purulent or mucopurulent secretions originating from the involved sinus cavity. All samples were obtained endonasally under endoscopic guidance from the sinus ostium or from the sinus cavity during surgery. Cultures were positive in 81.3 per cent of the control subjects and 83.1 per cent of the patients with rhinosinusitis. Corynebacteria, coagulase-negative staphylococci, propionibacteria and peptostreptococci were the main commensal organisms, while Haemophilus influenzae, streptococci, Streptococcus pneumoniae, Prevotella spp and Fusobacterium spp were probable causative pathogens. Anaerobes were isolated from approximately 25 per cent of the patients in the rhinosinusitis group. Betalactamase producers represented 27.5 per cent of H. influenzae and 28 per cent of Prevotella spp isolates. Diminished susceptibility to penicillin was found in 13 per cent of S. pneumoniae isolates. The amoxycillin-clavulanate combination was the most active oral antibiotic tested against the pathogenic species in vitro.