A retrospective survey of 147 patients with Down's syndrome (age range, 4 months to 27 years) showed that 60% had a thyrotropin (TSH) level higher than 5.7 mU/L in the presence of high or normal thyroxine levels. The remaining 40% of the group had low to normal TSH values. High TSH levels were predominant in patients under 4 years of age (94 children), ie, during the phase of active growth, and showed a declining trend with increasing age. All 94 infants had delayed growth of all parameters including head circumference, height, and weight, as compared with normal infants, and growth was particularly retarded in patients with TSH levels greater than 5.7 mU/L. Thyroid dysfunction, expressed as a high TSH concentration, is associated with growth retardation in children with Down's syndrome who are younger than 4 years.