BACKGROUND: Expert opinions vary on the appropriate role of antibiotics for sinusitis, one of the most commonly diagnosed conditions among adults in ambulatory care. OBJECTIVES: We examined whether antibiotics are effective in treating acute sinusitis, and if so, which antibiotic classes are the most effective. SEARCH STRATEGY: We searched the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library, 2007, Issue 3); MEDLINE (1950 to May 2007) and EMBASE (1974 to June 2007). SELECTION CRITERIA: Randomized controlled trials (RCTs) comparing antibiotics with placebo or antibiotics from different classes for acute maxillary sinusitis in adults. We included trials with clinically diagnosed acute sinusitis, whether or not confirmed by radiography or bacterial culture. DATA COLLECTION AND ANALYSIS: At least two review authors independently screened search results, extracted data and quality assessed trials. Risk ratios (RR) were calculated for differences in the intervention and control groups to see whether or not the treatment was a failure. In meta-analysing the placebo-controlled studies, the data across antibiotic classes were combined. Primary outcomes were the clinical failure rates at 7 to 15 days and 16 to 60 days follow up. MAIN RESULTS: Fifty-seven studies were included in the review; six placebo-controlled studies and 51 studies comparing different classes of antibiotics. Five studies involving 631 participants provided data for comparison of antibiotics to placebo, when clinical failure was defined as a lack of cure or improvement at 7 to 15 days follow up. These studies found a slight statistical difference in favor of antibiotics, compared to placebo, with a pooled RR of 0.66 (95% confidence interval (CI) 0.44 to 0.98). However, the clinical significance of the result is equivocal, also considering that cure or improvement rate was high in both the placebo group (80%) and the antibiotic group (90%). Based on six studies, when clinical failure was defined as a lack of total cure, there was significant difference in favor of antibiotics compared to placebo with a pooled RR of 0.74 (95% CI 0.65 to 0.84) at 7 to 15 days follow up. None of the antibiotic preparations was superior to each other. AUTHORS' CONCLUSIONS: Antibiotics have a small treatment effect in patients with uncomplicated acute sinusitis in a primary care setting with symptoms for more than seven days. However, 80% of participants treated without antibiotics improve within two weeks. Clinicians need to weigh the small benefits of antibiotic treatment against the potential for adverse effects at both the individual and general population level.