Background: We investigated muscle and plasma carnitine concentrations in children to establish reference intervals for use following biopsy of skeletal muscle. Methods: The study comprised 50 children from newborns up to 14 years of age, all undergoing elective surgery. They were divided into six age groups, the youngest 0-2 days and the oldest 11-14 years. The samples were taken at the beginning of surgery. Results: Gestational age was a major determinant of the total muscle carnitine concentration in newborns (Spearman's rs= 0.692, P<0.01). This concentration was low during the first year, but subsequently did not differ between age groups. In neonates the median value (range) for total carnitine concentration in skeletal muscle was 5.9 (2.2-15.9) µmol/g dry weight and the free to total carnitine ratio was 62 (31-81)%. In children 1-12 months old the corresponding figures were 6.0 (3.5-7.9) µmol/g dry weight and 51 (28-71)% and in those 1-14 years they were 12.1 (6.6-17.4) µmol/g dry weight and 76 (42-92)%. Conclusion: This study shows that muscle carnitine concentrations in newborns are dependent on gestational age. The data suggest that there is an accretion of carnitine in skeletal muscle during the first year of life. Reference intervals are given.