The aim of this systematic review was to synthesize the results of original studies assessing antibiotic efficacy at different time points after initiating treatment in patients with a moderate probability of acute bacterial rhinosinusitis. We searched the Cochrane library for systematic reviews on the efficacy of antibiotic treatment in patients with acute rhinosinusitis (ARS). Only randomized controlled trials (RCTs) that compared treatment of any antibiotic with placebo were included. The synthesis of the results of six RCTs showed a benefit of antibiotic treatment compared to placebo for the rate of improvement after 3 [pooled odds ratio (OR) 2.78 (95 % confidence interval (CI) 1.39–5.58)] and 7 [OR 2.29 (95 % CI 1.19–4.41)] days after initiation in patients with symptoms and signs of ARS lasting for 7 or more days. After 10 days [pooled OR 1.36 (95 % CI 0.66–2.90)], improvement rates did not differ significantly between patients treated with or without antibiotics. Compared to placebo, antibiotic treatment relieves symptoms in a significantly higher proportion of patients within the first days of treatment. Reporting an overall average treatment efficacy may underestimate treatment benefits in patients with a self-limiting illness.