Abstract 1. 1. Urine and serum samples from patients with ketoacidosis have been examined for organic acids by combined gas-liquid chromatography and mass spectrometry. 2. 2. Increased concentrations of 3-hydroxyisovaleric acid were found in all the urine samples from patients with ketoacidosis. A positive correlation was demonstrated to exist between the concentrations of 3-hydroxyisovaleric acid and 3-hydroxybutyric acid. 3. 3. It is proposed that a high concentration of acetoacetic acid and 3-hydroxybutyric acid, as well as a relatively increased oxidation of fatty acids, may influence the degradation of leucine and result in increased formation of 3-hydroxyisovaleric acid.