A pair of siblings with clinical symptoms of cyclic vomiting and ketoacidosis were found to have a biochemical triad of normoglycemia, ketoacidosis and elevated levels of alpha-hydroxy- and alpha-aminobutyrate in plasma and urine. Methionine loading studies in both sibs produced prompt rises in plasma methionine and alpha-aminobutyrate levels, with a subsequent increase in urinary alpha-hydroxybutyrate, as well. Leukocytes from both siblings showed normal oxidation of [3-14C]propionate. Increased inorganic sulfate excretion after methionine loading implied an intact transsulfuration pathway in both siblings. On the basis of the studies detailed in this report, we conclude that these siblings suffer from a defect in alpha-ketobutyrate oxidation, a newly described defect of organic acid metabolism.