In adult healthy men, a fast of 18 hours or greater produces ketonemia at a level that would result in detectable ketonuria. Aging is associated with increased susceptibility to fasting-induced hyperketonemia. (London, 1986) Ketonuria may be noted in normal pregnancy. (Chez, 1987) Acetoacetic acid, beta-hydroxybutyric acid, and acetone are ketone bodies. In ketosis, usually 80% of total ketones are beta-hydroxybutyric acid. Acetoacetic acid comprises most of the remainder with acetone present in trace amounts. Urine ketones should generally be determined in patients with a positive urine test for urine glucose and followed during the management of diabetes mellitus and ketoacidosis. Ketones can depress the glucose oxidase reaction, providing falsely low results, on some glucose oxidase reagent strips. In pregnancy, the risk of ketosis is increased; all pregnant type I diabetics are advised to monitor urine for ketosis in first morning urine and when blood glucose is >150 mg/dL. A portion of initial assessment for inborn errors of metabolism in infancy and childhood. In infants and children, ketonuria can occur with febrile illnesses and toxic states with marked vomiting or diarrhea. Genetic disorders resulting in ketonuria include propionyl CoA carboxylase deficiency, glycogen storage disease, branched-chain ketonuria, and methylmalonic aciduria.