Respiratory syncytial virus (RSV) bronchiolitis resulting in respiratory insufficiency is frequently encountered during the winter season in paediatric intensive care units. This review evaluates potential determinants described in severe RSV bronchiolitis with special attention to the role of immaturity of immune responses during infancy. Pre-existent cardiac or pulmonary compromises have been documented as clinical risk factors for severe RSV bronchiolitis. In addition to this group of infants with pre-morbidity, a large proportion of mechanically ventilated RSV bronchiolitis patients are previously healthy full-term infants or premature neonates without predisposing risk factors. In general, infants at this early age have maturation-related deficient cellular immunity. Several studies show an association between decreased cellular immunity and severe RSV bronchiolitis, indeed suggesting that a maturation-related defect of the cellular immune system facilitates severe RSV. In addition, low virus-specific antibody titres prior to RSV bronchiolitis have been shown to be a risk factor for severe RSV bronchiolitis. A low level of cellular and humoral immunity would explain that higher viral titres are found in infants with most severe illness. In conclusion, immaturity of the physiological functions in healthy pre-term infants and neonates, in particular the immune system, appears to be an important factor in the pathogenesis of RSV bronchiolitis resulting in mechanical ventilation.