We examined the effect of dietary carnitine on variables of lipid metabolism in human infants. Normal male full-term infants were fed an isolated soy-protein-based formula with or without added carnitine from age 6-9 d to age 112 d. Growth and food intake were measured throughout the study. At ages 56 and 112 d serum concentrations of carnitine, free fatty acids, and triglycerides and urinary excretion of carnitine and medium-chain dicarboxylic acids were measured. Serum carnitine concentrations were lower in all infants fed unsupplemented formula. There was no difference in growth or food intake between the two groups of infants. Serum free fatty acid concentrations were significantly higher in the infants not receiving dietary carnitine. Moreover, excretion of all three medium-chain dicarboxylic acids was significantly higher in infants not receiving dietary carnitine. We conclude that lack of dietary carnitine affects lipid metabolism of infants during the first 4 mo of life.