Functional residual capacity (FRC) was measured at 2–8 (median 4) weeks of age in 100 infants whose mothers had entered a randomized trial of amniocentesis versus chorion villus sampling (CVS) at 10–13 weeks gestation. At measurement all of the infants were well and none had respiratory problems. No significant difference was found in the FRC of infants whose mothers had undergone amniocentesis (n = 49) compared to those who had undergone CVS (n = 51). The 95% confidence intervals of the difference of the means of the amniocentesis group (29.7 ml/kg) and the CVS group (30.4 ml/kg) were ––1.17 and 2.52 ml/kg. Three patients in the amniocentesis group and 2 in the CVS group had an FRC < 24 ml/kg, which is below the 2.5th centile of the reference range. These preliminary results suggest that there is no difference in the effect on antenatal lung growth of these two first-trimester antenatal diagnostic pro