We have evaluated the thyroid function in 30 children and adolescents (average age 13 2/12 +/- 3 10/12 years) suffering from type 1 insulin-dependent diabetes (average duration of illness 5 +/- 3 3/12 years). We divided them into group A (good control) and group B (poor control), according to whether they had presented an HbA1C value lower or higher than 10%. The results obtained have been compared with a group of 30 normal patients of the same age. Both groups of diabetics presented T4 values which were notably lower than the control group (P less than 0.05). T3 was appreciably diminished both in group A (P less than 0.05) and in group B (P less than 0.01) and presented a negative correlation with HbA1C, only in group A however. The rT3/T3 ratio, on the other hand, was significantly increased in group B with respect to the controls. No significant differences were found with regard to FT3 and FT4, nor any correlations among thyroid hormones, C peptide, duration of illness, and daily insulin requirement. Our results indicated that the tendency to the low T3 syndrome, already described in adult diabetics, is also identifiable in young diabetics, particularly if poorly controlled.