Telithroniycin (HMR 3647) is a new ketolide antimicrobial that was developed for the treatment ofcommunityacquired respiratory tract infections. We conducted a randomized, double-blind, multicenter study to compare the clinical efficacy and safety of oral telithromycin; at 800 mg once dail yfor 5 or 10 days, with that ofamoxicillin/ clavulanic acid, at 500/125 mg three times daily for 10 days, in adults with acute maxillary sinusitis (AMS). A total of 754 patients with AMS of less than 28 days' duration were randomized to receive either telithromycin for 5 days follow ed by pla cebo for 5 days, telithromycin fo r 10 days, or amoxicillin/clavulanic acidfor 10 days. Clinical out com e was assessed at a test-of-cure (TOC) visit between days 17 and 24 and at a late post-therapy visit between days 31 and 45. Analysis ofclinical outcome on a per-protocol basis (n =434) demonstrated therapeutic equivalence among the three regimens at the TOC visit; in each treatment group , the clini cal cure rate was approximately 75%. Only a few patients (3 to 5 in each group) had relapsed by the late post-therapy follow-up visit. Telith romycin was generally saf e and well tolerated. The most common adverse events were mild or moderate gastrointestinal effects, which occurred with similarfrequency in all three group s. We conclude that 5 or 10 days of telithromycin at 800 mg once daily is as effective clinically and as well tolerated as 10 days of