Diabetic ketoacidosis is a serious complication of type diabetes. beta-Hydroxybutyrate (beta-OHB) accounts for about 75% of ketones, and blood concentration can be determined with a sensor. The aim of this study was to investigate the frequency and degree of ketonemia in daily life of children with diabetes and to make a base for recommendations for determination of ketonemia in clinical practice. During 3 months 45 patients with type 1 diabetes since 1-10 years old (mean 4.4 +/- 3.3 years old) at the pediatric clinic in Linköping, Sweden, performed 24-h profiles (eight determinations) in 2 weeks with blood glucose and beta-OHB. The children performed 11,189 blood glucose and 7,057 beta-OHB measurements. Only 0.3% (n = 21) of beta-OHB measurements were > or = 1.0 mmol/L. An beta-OHB concentration > 0.2 mmol/L was more common in the morning than during the rest of the day (p < 0.001). Young children (4-7 years old) had values > or = 0.2 mmol/L more often than adolescents (p < 0.001). Blood glucose values > 15 mmol/L were more often accompanied by beta-OHB > 0.2 mmol/L (p < 0.001). High beta-OHB concentrations are rare in diabetic children with reasonably good metabolic control. Already a value > 0.4 mmol/L seems abnormal, and we recommend that patients retest glucose and ketones with beta-OHB > 0.4 mmol/L. Furthermore, we recommend that diabetic children and adolescents measure beta-OHB when symptoms like nausea or vomiting occur to differentiate ketoacidosis from gastroenteritis, and during infections, during periods with high blood glucose (> 15 mmol/L), and if they notice ketonuria. Monitoring beta-OHB should be routine for patients on pump therapy.