The variations in blood ketone bodies, blood glucose, and insulin were studied in 19 normal and 14 hypoglycemic children, 4 months to 13 years of age, during a 24-hour fast. Except in four patients (two with hyperinsulinism and two with congenital defect in ketogenesis), a significant increase in blood ketone bodies was observed in both controls and patients. A progressive decrease in glucose concentrations was observed up to but not after 20 hours. A highly negative correlation between blood ketone bodies and blood glucose was found, with a large dispersion of blood ketone bodies, especially for those corresponding to the blood glucose between 45 and 65 mg/dl. This dispersion was consistently reduced in a homogenous age group of 4 to 6 years with similar glucose values. There was a positive correlation between age and blood glucose from hour 21 on, and an inverse relationship between age and blood ketone bodies from hour 15 on. The same high inverse relationship between age and blood ketone bodies was again observed when the variable of glucose concentration was factored out, demonstrating that the variation in blood ketone bodies is indeed related to age. These findings need to be taken into account in the interpretation of fasting blood ketone bodies, especially when used as an aid in the diagnosis of the various forms of childhood hypoglycemia, and of hypoketotic states.