We quantitated serial serum beta-hydroxybutyrate (beta-OHB) levels using the Ketosite method in 9 children with IDDM who were treated for diabetic ketoacidosis (DKA) and compared them to urinary ketone measurements by dipstick. Persistent elevations of serum beta-OHB were seen in six patients when the urine became clear of ketones; five of these patients had a recurrence of ketonuria. We conclude that many patients recovering from ketoacidosis have continuing elevations of beta-OHB after the urine is free of ketones and this unrecognized abnormality is the likely cause of recurrence of the ketonuria. We recommend that fluid therapy be continued beyond clearance of ketonuria and suggest using the Ketosite method to document restoration of normal serum beta-OHB levels in patients recovering from DKA.