There is a relationship between the fatty acid profile in skeletal muscle phospholipids and peripheral resistance to insulin in adults, but similar data have not been reported in infancy and childhood. The objective of this study was to investigate the fatty acid composition of skeletal muscle and adipose tissue across the paediatric age range. The fatty acid profile of skeletal muscle phospholipids and adipose tissue triacylglycerols was analysed in ninety-three healthy Spanish infants and children distributed into four groups: group 1 (0 to <2 years, n 10); group 2 (2 to <5 years, n 41); group 3 (5 to <10 years, n 24); group 4 (10 to 15 years, n 18). In skeletal muscle phospholipids, oleic acid (18: 1n-9cis) content decreased significantly whereas that of linoleic (18: 2n-6) acid increased significantly with age (P for trend <0·01). In adipose tissue, the contents of triacylglycerol and linoleic acid increased significantly across the paediatric age range (P for trend <0·01), whereas dihomo-γ-linolenic (20: 3n-6) and arachidonic (20: 4n-6) showed significant differences between groups. The variations in fatty acid composition observed with age indicated an imbalance in dietary n-3/n-6 long-chain PUFA.