We studied concentrations of thyroid hormones (T3, T4, FT4, rT3, TBG and TSH) in 62 type I diabetic children and adolescents. The patients were classified into group A (n = 27, good control, HbA1c less than 10%), group B (n = 19, poor control, HbA1c greater than 10%) and group C (n = 16, diabetic ketoacidosis, pH less than 7.1 and HCO3 less than 15 mmol/L. All patients were treated with two daily injections of purified monocomponent insulins. Thirty healthy subjects of the same age served as control group. Patients in group B and C had significantly lower T3 and higher rT3 levels (p less than 0.001) compared to the matched controls (1.5 vs 2.2; 0.9 vs 2.2; 0.58 vs 0.3 and 0.6 vs 0.3 nmol/L). Serum TBG levels were significantly lower (p less than 0.01) in the group A (19.5 +/- 4.3 mg/L), group B (20.3 +/- 3.3) and group C (18.0 +/- 3.4) compared with control group (24.2 +/- 3.1). There was significantly negative correlation between T3 and HbA1c in group B (r = 0.546; p less than 0.02). The results of this study confirm that euthyroid sick syndrome does exist in type I diabetic children and adolescents with poor metabolic control and ketoacidosis. The inverse relationship between T3 and HbA1c percentage (low T3 and high HbA1c) points to the poor diabetic control.