BACKGROUND: Croup (laryngotracheobronchitis) is a common cause of upper airway obstruction in children with a peak incidence of 60 per 1000 child years in those aged between one and two years. It is characterised by hoarseness, a barking cough, and inspiratory stridor. These symptoms are thought to occur as a result of oedema of the larynx and trachea, which have been triggered by a recent viral infection. Para influenza virus type 1 is the agent most commonly identified in cases of croup. Severe cases are admitted to hospital and steroid treatment is established to reduce disease severity. Treatment with humidified air was previously widely used and is still commonly recommended as home treatment. OBJECTIVES: To assess the efficacy of humidified air in the treatment of croup. SEARCH STRATEGY: We searched the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library Issue 4, 2005), MEDLINE (1966 to January 2006) and EMBASE (1990 to January 2006). SELECTION CRITERIA: Randomised controlled trials (RCTs) involving children suffering from croup treated with humidified air. DATA COLLECTION AND ANALYSIS: Two authors independently identified potentially relevant abstracts identified from the search and then assessed the full papers for inclusion and methodological quality. Outcome measures included mortality, ventilation, admission to hospital, re-contact with medical services, number of days off school and relief of symptoms; these were separately analysed for the week following treatment. Data extraction was performed by the two authors then entered by one and checked by the second author. Missing data were obtained from trails authors where possible. Data were analysed using Review Manager version 4.2. Sensitivity and sub-group analysis were not possible due to the paucity of trials. MAIN RESULTS: Three studies in emergency settings provided data on 135 patients with moderate croup for the main outcome (croup score). The combined results from 20 to 60 minutes in the three studies marginally favoured the treatment group with a weighted standardised mean difference of -0.14 (95% confidence interval (CI) -0.75 to 0.47). No other outcomes were significantly different between the groups. AUTHORS' CONCLUSIONS: The croup score of children managed in an emergency setting with mild to moderate croup probably does not improve greatly with inhalation of humidified air. Further research is needed in primary care settings, using a wider range of more sensitive outcome measures.