• We examined the relationship between serum β-hydroxybutyrate (BOHB) and plasma glucose concentrations and between serum free fatty acid (FFA) and glucose concentrations in 34 normal children who fasted for up to 24 hours. The BOHB concentration correlated inversely with the glucose concentration, as did the FFA concentration. We compared these results with those in six patients with hypoglycemia due to hyperinsulinism. In the hyperinsulinemic children, hypoglycemia was invariably associated with relative hypoketonemia (glucose concentration, <40 mg/dL; BOHB concentration, <2mM; >2 SDs below the mean). The FFA concentrations were also unduly low (>2 SDs). However, despite being significantly hypoketonemic when hypoglycemic, four of the six hyperinsulinemic patients had ketonuria. Since it does not exclude hyperinsulinism, ketonuria may be a potentially misleading finding in a hypoglycemic child.