Aseptic samples of sinus mucosa or aspirates from maxillary sinuses were taken from 197 patients who underwent sinus surgery and were cultured for aerobic bacteria. Anaerobic cultures were also obtained from 143 of these patients. A total of 236 bacterial isolates from 174 patients were analyzed. No anaerobic organisms were isolated. The most commonly isolated organisms were Staphylococcus species (coagulase-negative), Staphylococcus aureus, gram-negative bacilli, and Streptococcus species. Penicillin G, erythromycin, tetracycline hydrochloride and, to a lesser degree, first-generation cephalosporins were found to be inferior to ciprofloxin, trimethoprim/sulfamethoxazole, and cefuroxime axetil when the susceptibilities of all the bacterial isolates to oral antibiotics used commonly in the empiric therapy of chronic sinusitis were compared.