Aim: To examine over time, the cellular response within the lungs of infants ventilated with respiratory syncytial virus (RSV) bronchiolitis and to compare this response in infants born at term with those born preterm.

Methods: Non-bronchoscopic bronchoalveolar lavage (BAL) samples were taken from 47 infants (24 born at term and 23 born preterm) who were ventilated for RSV positive bronchiolitis and 10 control infants. BAL cellularity and differential cell counts were calculated using standard techniques.

Results: Total cellularity in BAL over the first four days of ventilation in infants with RSV bronchiolitis was greater in term infants (median 2.2 (IQR 4.27) × 10⁶ cells/ml) compared with preterm infants (0.58 (1.28) × 10⁶ cells/ml). The magnitude of the cellular response in preterm infants with bronchiolitis was similar to that in the control group measured on day 1 (0.62 (0.77) × 10⁶ cells/ml). BAL cellularity decreased progressively from the time of intubation in term infants, but remained relatively constant in preterm infants up to seven days after intubation.

Conclusions: There are differences in the magnitude and type of pulmonary cellular response in term and preterm infants ventilated with RSV bronchiolitis. The cellular response in term infants with bronchiolitis differs from that in a control group of infants. These differences may reflect variations in cellular recruitment in the lung and/or variations in airway calibre.