Telithromycin (HMR 3647) is a new ketolide antimicrobial that was developed for the treatment of community-acquired respiratory tract injections. We conducted a randomized, double-blind, multicenter study to compare the clinical efficacy and safety of oral telithromycin, at 800 mg once daily for 5 or 10 days, with that of amoxicillin/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 followed by placebo for 5 days, telithromycin for 10 days, or amoxicillin/clavulanic acid for 10 days. Clinical outcome 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 of clinical outcome on a per-protocol basis (n = 434) demonstrated therapeutic equivalence among the three regimens at the TOC visit; in each treatment group, the clinical 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. Telithromycin was generally safe and well tolerated. The most common adverse events were mild or moderate gastrointestinal effects, which occurred with similar frequency in all three groups. 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 treatment with amoxicillin/clavulanic acid. Telithromycin, therefore, appears to be a valuable option for the treatment of AMS.