Abstract: Background The benefits and risks of antibiotics for acute bronchitis remain unclear despite it being one of the most common illnesses seen in primary care. Objectives To assess the effects of antibiotics for patients with a clinical diagnosis of acute bronchitis. Search methods We searched the Cochrane Central Register of Controlled trials (CENTRAL 2010, Issue 3) including the Acute Respiratory Infections (ARI) Group's Specialised Register, MEDLINE (1966 to August Week 4, 2010) and EMBASE (1974 to September 2010). Selection criteria Randomised controlled trials (RCTs) comparing any antibiotic therapy with placebo or no treatment in acute bronchitis or acute productive cough, without other obvious cause, in patients without underlying pulmonary disease. Data collection and analysis At least two review authors extracted data and assessed trial quality. We contacted trial authors for missing data. Main results Fifteen trials with 2618 patients, that included smokers and non‐smokers, were included in the primary analysis. The quality of trials was generally good, particularly for more recent studies. There was limited evidence to support the use of antibiotics in acute bronchitis. At follow‐up, patients receiving antibiotics were marginally more likely to be clinically improved than those receiving placebo treatment (nine studies with 1754 patients, risk ratio (RR) 1.06; 95% confidence interval (CI) 1.02 to 1.10). At follow‐up, patients given antibiotics were less likely to have a cough (four studies with 275 participants, RR 0.64; 95% CI 0.49 to 0.85; number needed to treat for an additional beneficial outcome (NNTB) 6); have a night cough (four studies with 538 participants, RR 0.67; 95% CI 0.54 to 0.83, NNTB 7); not improve according to the clinician's global assessment (six studies with 891 participants, RR 0.61; 95% CI 0.48 to 0.79, NNTB 25); and have an abnormal lung exam (five studies with 613 participants, RR 0.54; 95% CI 0.41 to 0.70, NNTB 6). Antibiotic‐treated patients also had a reduction in days feeling ill (five studies with 809 participants, mean difference (MD) ‐0.64; 95% CI ‐1.16 to ‐0.13) and a reduction in days with limited activity (six studies with 767 participants, MD ‐0.49; 95% CI ‐0.94 to ‐0.04). The differences in presence of a productive cough at follow‐up, proportions with activity limitations at follow‐up, mean duration of cough and mean duration of productive cough did not reach statistical significance. There was a non‐significant trend towards an increase in adverse effects in the antibiotic group (10 studies with 1509 participants) (RR 1.15; 95% CI 0.92 to 1.44). Authors' conclusions There is limited evidence to support the use of antibiotics in acute bronchitis. Antibiotics may have a modest beneficial effect in some patients with acute bronchitis though data on subsets of patients who may benefit more from treatment is lacking. However, the magnitude of this benefit needs to be considered in the broader context of potential side effects, medicalisation for a self limiting condition, increased resistance to respiratory pathogens and cost of antibiotic treatment.