The excretion of 2-(2'-octenyl)succinic acid (OSA) and several metabolites of OSA was studied by gas chromatography/mass spectrometry in 17 infants and children fed one of three proprietary elemental or protein-hydrolysate formulas that use OSA-modified cornstarch as an emulsifying agent. Variable but often large amounts (up to 2500 mg/g creatinine) of the fatty acid-like OSA and its metabolites were found in the urine of these children, and levels of OSA in their blood ranged from 9.5 to 57.9 mumol/L. Apparently secondary abnormalities, such as increased urinary levels of glutaric acid and 2-ketoglutaric acid, were also found in more than half of the urine specimens. The molecular weight and mass fragmentation patterns of the nine compounds associated with the excretion of OSA are consistent with the proposal that OSA is metabolized in human infants and children by a combination of omega-, omega-1-, and beta-oxidation steps, similar to the metabolism of another branched-chain fatty acid, valproic acid. The urinary organic acid pattern of children fed elemental formulas containing OSA-modified starch often was dominated by OSA and its metabolites, and in several children the OSA-related changes were mistaken for a primary metabolic disease. Physicians and laboratories evaluating children for suspected metabolic diseases should be aware of the possibility of abnormal organic acid studies associated with OSA-containing formulas.