Although it is known that culture-directed therapy along with surgery increases treatment success; the antimicrobial therapy is usually selected on empirical basis, mostly because of the lack of time and equipment for a proper endoscopy. Therefore, most of the centers follow up the treatment protocols directed against the known sinus pathogens. The purpose of this study was to characterize the bacteriology of outpatient chronic maxillary sinusitis, and argue about the effectiveness of known antimicrobial agents against these microorganisms. A prospective study was carried out on 44 consecutive patients referred to an outpatient clinic at The Kocaeli University Hospital, Department of Oto-Rhino-Laryngology, between May 1999 and January 2000. Swabs of 44 chronically inflamed maxillary sinuses were processed for aerobic bacteria as well as yeast and yeast-like organisms during endoscopy. Antibiograms were carried using Kirby-Bauer disk diffusion technique. Of the 44 total cultures, there were 38 (86.3%) positive cultures yielding 49 isolates. Thirty-two cultures had two or fewer isolates; two cultures had three isolates. The most frequently isolated organisms were coagulase-negative staphylococcus ( 21 of 49, 42.8%), Streptococcus spp. (12 of 49, 24.4%), Staphylococcus aureus (8 of 49, 16.3%), Pseudomonas aeruginosa (5 of 49, 10.2%) and Candida albicans ( 3 of 49, 6.1%). According to antimicrobial susceptibility testing, therapy in protocol with amoxicillin-clavulanic acid was shifted to an another antibiotic in 24 of the 44 cases (54.5%). Most of the therapy protocols on chronic sinusitis are based on the studies in which Streptococcus pneumoniae and Haemophilus influenzae are known as the most important pathogens. But as Staphylococcus spp. accounted for more than 50% of cases in this study, beta-lactam antimicrobials such as amoxicillinclavulanate which shows the best activity against pneumococci, Haemophilus influenzae and Moraxella catarrhalis, would not be an appropriate agent and should not be selected as the first drug. In a conclusion, antimicrobial treatment should be based on serial culture and sensitivity results and coordinated with surgical drainage.