Bronchiolitis is a serious, potentially life-threatening respiratory illness commonly affecting babies. It is often caused by respiratory syncytial virus (RSV). Antibiotics are not recommended for bronchiolitis unless there is concern about complications such as secondary bacterial pneumonia or respiratory failure. Nevertheless, they are used at rates of 34% to 99% in uncomplicated cases. To evaluate the effectiveness of antibiotics for bronchiolitis. We searched the Cochrane Central Register of Controlled Trials (CENTRAL 2010, issue 4), which includes the Cochrane Acute Respiratory Infection Group's Specialised Register, and the Database of Abstracts of Reviews of Effects, MEDLINE (January 1966 to November 2010), EMBASE (1990 to December 2010) and Current Contents (2001 to December 2010). Randomised controlled trials (RCTs) comparing antibiotics to placebo in children under two years diagnosed with bronchiolitis, using clinical criteria (including respiratory distress preceded by coryzal symptoms with or without fever). Primary clinical outcomes included time to resolution of signs or symptoms (pulmonary markers included respiratory distress, wheeze, crepitations, oxygen saturation and fever). Secondary outcomes included hospital admissions, length of hospital stay, re-admissions, complications or adverse events and radiological findings. Two review authors independently analysed the search results. Five studies (543 participants) met our inclusion criteria. One study randomised 52 children to either ampicillin or placebo and found no significant difference between the two groups for length of illness. A small study (21 children) with higher risk of potential bias randomised children with proven RSV infection to clarithromycin or placebo and found clarithromycin may reduce hospital re-admission (8% antibiotics versus 44% placebo; Fishers exact; P = 0.081). The two studies (267 children) providing adequate data for length of hospital stay showed no difference between antibiotics and control (pooled mean difference 0.34; 95% CI -0.71 to 1.38). Two studies randomised children to intravenous ampicillin, oral erythromycin and control and found no difference for most symptom measures. None of the trials reported deaths. This review found minimal evidence to support the use of antibiotics for bronchiolitis. Research to identify a possible small subgroup of patients who have complications from bronchiolitis such as respiratory failure and who may benefit from antibiotics is justified.