Comparisons of the components of ventilator waveform were made in two groups of preterm infants ventilated for the respiratory distress syndrome. Infants actively expiring against positive pressure inflation were compared to infants not showing this interaction. The actively expiring infants were ventilated with a higher peak inspiratory pressure (P less than 0.01) and a significantly greater duration of positive pressure plateau (P less than 0.05) than the control group. The two groups were well matched for gestational and postnatal age and there were no other significant differences in the components of ventilator waveform. Adoption of a ventilator waveform with a shorter positive pressure plateau and less steep rise in positive pressure might reduce the incidence of active expiration and hence pneumothoraces.