The role of carnitine in oxidation of dietary medium-chain fatty acids (as medium-chain triglycerides) was studied in term human infants. Infants were fed, alternately, formulas with fat content that was predominantly long-chain triglycerides, or 40% medium-chain triglycerides. Urinary acylcarnitine excretion was significantly higher and the ratio of free to total carnitine was significantly lower when infants were fed the formula with medium-chain triglycerides. Two groups of 10 infants were fed a commercial soy-protein-based formula modified to contain 40% of fat calories as medium-chain triglycerides and with or without added L-carnitine. By 56 d, infants fed the formula without added L-carnitine excreted significantly more medium-chain dicarboxylic acids than did the same infants at 28 d and significantly more than infants consuming the carnitine-supplemented formula at either 28 or 56 d. Results are consistent with a role for carnitine in metabolism of dietary medium-chain triglycerides in infants.