In a boy with a defect in fatty acid oxidation due to the hepatic form of carnitine palmitoyltransferase-1 deficiency, plasma carnitine concentrations were found to be twice normal. The elevation in plasma carnitine levels was accompanied by an unusually high renal threshold for free carnitine, suggesting a secondary increase in carnitine transport. Similar to other fatty acid oxidation disorders involving the carnitine cycle, urinary dicarboxylic acids were not abnormally elevated during illnesses. The combination of elevated plasma carnitine levels and absence of dicarboxylic aciduria may help to distinguish the hepatic form of carnitine palmitoyltransferase-1 deficiency from other defects in fatty acid oxidation.