A randomized, double-blind, multicenter study of adults with acute bacterial sinusitis (ABS) compared the efficacy and safety of two azithromycin (AZM) regimens, 500 mg/day once daily for 3 days (AZM-3) or 6 days (AZM-6) to the efficacy and safety of an amoxicillin-clavulanate (AMC) regimen of 500-125 mg three times daily for 10 days. A total of 936 subjects with clinically and radiologically documented ABS were treated (AZM-3, 312; AZM-6, 311; AMC, 313). Clinical success rates were equivalent among per-protocol subjects at the end of therapy (AZM-3, 88.8%; AZM-6, 89.3%; AMC, 84.9%) and at the end of the study (AZM-3, 71.7%; AZM-6, 73.4%; AMC, 71.3%). Subjects treated with AMC reported a higher incidence of treatment-related adverse events (AE) (51.1%) than AZM-3 (31.1%, P < 0.001) or AZM-6 (37.6%, P < 0.001). More AMC subjects discontinued the study (n = 28) than AZM-3 (n = 7) and AZM-6 (n = 11) subjects. Diarrhea was the most frequent treatment-related AE. AZM-3 and AZM-6 were each equivalent in efficacy and better tolerated than AMC for ABS.