In order to delineate the spectrum of thyroid abnormalities in children with Down's syndrome (DS), first visit height data (SDS) and serum TSH, T4 and antiperoxidase antibodies concentrations were retrospectively evaluated in 137 children (71 girls) with DS (0.04-16 years). Congenital hypothyroidism was detected in 2.9% of patients. Thyroid disease occurred in 9%: four hyperthyroidism and eight hypothyroidism. Overt thyroid disease was always related to thyroid autoimmunity. The remaining 121 patients had normal T4 levels but increased mean TSH compared with controls (4.7 +/- 2.8 vs 2.3 +/- 1.3 mU/l). According to TSH levels, they were divided into two groups: G1 (n = 68) with normal TSH (<5 mU/l), and G2 (n = 53) with high TSH (> 5 mU/l). T4 levels were significantly lower in G2 (p < 0.01 vs G1 and controls). Height SDS was not different. Thyroid disorders are frequent in children with DS. Subtle thyroid abnormalities found in patients with DS with no evidence of clinical dysfunction need further investigation to demonstrate whether there is a need for therapeutic intervention.