Respiratory function was assessed at a median of 7 months (range 6-12) in 17 preterm infants who, in the neonatal period, had been entered into a multi-centre randomized placebo-controlled trial of prophylactic surfactant replacement therapy. Seven infants (median gestational age 28 weeks) received surfactant and the remaining ten infants (median gestational age 27 weeks) placebo. Respiratory function was assessed by measuring functional residual capacity (FRC), thoracic gas volume (TGV) and airways resistance (RAW). Specific conductance (SGAW) was calculated from RAW and TGV. There was no significant difference in FRC or TGV between the two groups. RAW, however, was significantly lower in the surfactant (median 41, range 21-48 cmH2O l-1 s-1) compared to the placebo group (median 57, range 40-68 cmH2O l-1 s-1), P < 0.05 and SGAW significantly higher in the surfactant (median 0.136, range 0.063-0.289 l cmH2O-1 s-1) compared to the placebo group (median 0.081, range 0.062-0.134 l cmH2O-1 s-1), P < 0.05. These results suggest that surfactant replacement therapy improves lung function at follow-up.