INTRODUCTION: Bronchiolitis is the most common lower respiratory tract infection in infants, occurring in a seasonal pattern, with highest incidence in the winter in temperate climates, and in the rainy season in warmer countries. Bronchiolitis is a common reason for attendance at and admission to hospital. METHODS AND OBJECTIVES: We conducted a systematic review and aimed to answer the following clinical questions: What are the effects of prophylactic interventions for bronchiolitis in high-risk children? What are the effects of measures to prevent transmission of bronchiolitis in hospital? What are the effects of treatments for children with bronchiolitis? We searched: Medline, Embase, The Cochrane Library and other important databases up to October 2006 (Clinical Evidence reviews are updated periodically, please check our website for the most up-to-date version of this review). We included harms alerts from relevant organisations such as the US Food and Drug Administration (FDA) and the UK Medicines and Healthcare products Regulatory Agency (MHRA). RESULTS: We found 40 systematic reviews, RCTs, or observational studies that met our inclusion criteria. CONCLUSIONS: In this systematic review we present information relating to the effectiveness and safety of the following interventions: bronchodilators (oral, inhaled salbutamol, inhaled adrenaline [epinephrine]), chest physiotherapy, corticosteroids, montelukast, nursing interventions (cohort segregation, hand washing, gowns, masks, gloves, and goggles), respiratory syncytial virus immunoglobulins, pooled immunoglobulins, or palivizumab (monoclonal antibody), ribavirin, or surfactants.