Determining bacterial aetiology of acute sinusitis is difficult without employing invasive procedures. Most episodes of acute sinusitis resolve spontaneously. Antibiotics have demonstrated efficacy for the treatment of acute bacterial sinusitis in clinical trials yet little is known of their effectiveness in real-life treatment settings. WHAT THIS STUDY ADDS Most cases of untreated acute sinusitis resolved spontaneously. Antibiotics were more effective when given within the first 10 days of treatment. This had no effect on later recurrence. Patients with poor oro-dental condition or recent antibiotic use may derive the most benefit from an antibiotic prescription and this should be considered by prescribers. The antibiotics used were found to be equally effective. Existing recommendations to identify acute sinusitis with high probability of bacterial origin, such as the French recommendations, fever or duration of symptoms fail to identify patients in whom antibiotics are more effective. AIMS To assess the effectiveness of antibiotics in acute bacterial sinusitis. METHODS This was a prospective cohort study with 2 months follow-up of 5640 patients with acute sinusitis included by a random sample from 1174 GPs and 120 ENT specialists. Main outcomes were short-term initial success, defined as the absence of prescription of (another) antibiotic or sinus lavage within 10 days, and lack of recurrence between the 11th and 60th day, after initial success. RESULTS Initial success was found in 88.7% (95% CI 85.1, 91.4%) of patients without antibiotic prescription at inclusion and 96.2% (95% CI 95.7, 96.7%) of patients prescribed antibiotics. The 10 day adjusted hazard ratio (HR) for treatment failure (new antibiotic prescription or sinus drainage) with initial antibiotics compared with no antibiotics was 0.30 (95% CI 0.21, 0.42) with no difference between antibiotics. Antibiotics were more effective in patients with poor oro-dental condition (HR 0.04, 95% CI 0.01, 0.20) and in patients who had already used antibiotics during the previous 2 months (HR 0.09, 95% CI 0.03, 0.28). For patients without failure at 10 days, recurrence between the 11th and 60th day was similar whether or not they had initially been prescribed an antibiotic, 94.1% (95% CI 93.4, 94.7%) and 93.4% (95%CI 90.3, 95.5%), respectively. CONCLUSION Most acute sinusitis cases not prescribed antibiotics resolve spontaneously. Antibiotics reduced by 3.3-fold the risk of failure within 10 days, without impact on later recurrence. The greatest benefit of antibiotics was found for patients with poor oro-dental condition or with antibiotic use within the previous 2 months.