Antenatal carnitine administration has been shown to induce fetal lung maturity by increasing pulmonary surfactant in animal and human studies. The aim of this study was to investigate serum free carnitine (FC) levels in preterm infants with respiratory distress syndrome (RDS) and controls during the first week of postnatal life. The study groups consisted of 76 preterm infants with gestational ages ranging from 28 to 36 weeks, and birthweights ranging from 1046 to 2352 g. Serum FC levels were measured in preterm infants (group A, 37 with RDS; group B, 39 controls without RDS) within the first 6 hours after birth, on days 3 and 7. For specific analyses, serum FC levels were determined for gestational ages 28 to 31 weeks and 32 to 36 weeks in both groups. Initial FC levels were decreased insignificantly in group A (22.5 +/- 7.3 mu mol/L) compared with group B (23.5 +/- 6.8 mu mol/L; p > 0.05). On days 3 and 7 of life, serum FC levels were significantly lower in group A (18.3 +/- 6.1 and 10.2 +/- 3.3 mu mol/L, respectively) than in group B (23.4 +/- 7.1 and 22.8 +/- 3.7 mu mol/L, respectively; p < 0.05 and p < 0.05, respectively) on days 3 and 7 of life, respectively. Serum FC level remained stable in the non-RDS group (p > 0.05), but it decreased significantly in the RDS group during the first week of postnatal life (P < 0.05). No differences were seen between the corresponding gestational age groups. Serum FC levels in RDS infants decreased from days 1 to 7. Decreased neonatal serum carnitine levels in preterm infants with RDS during the first week of life might be caused by increasing consumption of carnitine in lung tissue for surfactant synthesis.