BACKGROUND Most clinicians prescribe antibiotics for acute bronchitis in spite of recommendations against this practice. Because the results of individual clinical trials have been mixed, we conducted a meta-analysis to determine whether antibiotics are effective treatment for acute bronchitis. METHODS We conducted a comprehensive search to identify all trials in which patients who had a diagnosis of acute bronchitis were randomly assigned to treatment with an antibiotic or placebo. Patient-oriented outcomes of importance that were reported in at least 3 studies were quantitatively summarized. RESULTS Nine studies met the inclusion criteria, but not all trials provided data for each outcome. Patients given antibiotics were less likely to have a cough (relative risk [RR] = 0.69; 95% confidence interval [CI], 0.49 -0.98) and be considered unimproved (RR = 0.51; 95% CI, 0.30-0.88) at a follow-up visit; but they were not less likely to have a productive cough (RR = 0.79; 95% CI, 0.60-1.03), activity limitations (RR = 0.59; 95% CI, 0.24-1.44), or feel ill (RR = 0.70; 95% CI, 0.31-1.58). Antibiotic-treated patients had a slightly shorter duration of productive cough (weighted mean difference [WMD] = -0.56 days; 95% CI, -1.09 to -0.04), but not of overall cough (WMD = -0.94; 95% CI, -2.08 to 0.21) or activity limitations (WMD = -0.49; 95% CI, -1.07 to 0.10). Patients treated with antibiotics did not report significantly more adverse effects (RR = 1.47; 95% CI, 0.82-2.65). CONCLUSIONS Antibiotics may be modestly effective for a minority of patients with acute bronchitis. It is not clear which patient subgroups might benefit, and the failure of some studies to report negative findings may have resulted in overestimates of the benefits of antibiotics. Antibiotics are not necessary for every patient with acute bronchitis.