Respiratory syncytial virus (RSV) bronchiolitis is a common infection in young children and may result in hospitalization. We examined the incidence of, and risk factors associated with, hypoxemia and respiratory failure in 216 children aged < 24 months admitted consecutively for proven RSV bronchiolitis. Hypoxemia was defined as SpO2 < 90% in room air and severe RSV bronchiolitis requiring intubation and ventilation was categorized as respiratory failure. Corrected age at admission was used for premature children (gestation < 37 weeks). Hypoxemia was suffered by 31 (14.3%) children. It was more likely to occur in children who were Malay (OR 2.56, 95%CI 1.05-6.23, p=0.03) or premature (OR 6.72, 95%CI 2.69-16.78, p<0.01). Hypoxemia was also more likely to develop in children with failure to thrive (OR 2.96, 95%CI 1.28-6.82, p<0.01). The seven (3.2%) children who were both premature (OR 11.94, 95%CI 2.50-56.99, p<0.01) and failure to thrive (OR 6.41, 95%CI 1.37-29.87, p=0.02) were more likely to develop respiratory failure. Prematurity was the only significant risk factor for hypoxemia and respiratory failure by logistic regression analysis (OR 1.17, 95%CI 1.06-1.55, p<0.01 and OR 1.14 95%CI 1.02-2.07, p=0.02 respectively). Prematurity was the single most important risk factor for both hypoxemia and respiratory failure in RSV bronchiolitis.