We have investigated if respiratory distress syndrome (RDS) treated by an increased inspired oxygen concentration, rather than mechanical ventilation, was associated with impaired lung function at follow-up and/or an increase in respiratory symptoms. Thoracic gas volume (TGV) and airways resistance (RAW) were measured in eight pre-term infants (median gestational age 29 weeks) at 6 and 12 months of age. The infants had suffered from RDS but had not required mechanical ventilation. Their results were compared to 16 other infants, matched for gestational age; eight who had required ventilation in the neonatal period and eight who had had no RDS. In all three groups the occurrence of respiratory symptoms was recorded. The lung function of the infants requiring oxygen in the neonatal period was similar to those who had not suffered from RDS, but their airways resistance was significantly lower at 6 but not 12 months than that of infants ventilated in the neonatal period (P less than 0.05). There was no significant difference in recurrent respiratory symptoms between the three groups although a greater proportion of the infants ventilated in the neonatal period were symptomatic in the first 6 months of life. These results suggest that oxygen therapy alone does not result in an impairment of lung function which is independent of the effect of prematurity.