Length/height was studied from birth to 6 years of age in 103 children with congenital hypothyroidism identified by the Norwegian or Swedish screening programs. We used the "infancy-childhood-puberty (ICP) growth model". This model describes normal linear growth during the first 3 years of life by an infancy component with the addition of a childhood component, the latter acting from the second half of the first year. In comparison with reference children, children with hypothyroidism had reduced growth from 6 to 12 months, and increased growth after 12 months of age. Mean onset of the childhood component of growth was delayed from 8.1 months (SD 1.9) to 10.4 months (SD 2.2) in girls, and from 8.9 months (SD 2.0) to 11.0 months (SD 2.1) in boys. Age at onset of the childhood component was correlated with age at start of treatment (r = 0.24), and in children with more severe hypothyroidism (pretreatment serum thyroxine < 40 nmol/l) inversely correlated with the L-thyroxine dose at start of treatment (r = -0.40). Change in height standard deviation score from 1 to 3 years of age was correlated with the serum thyroxine concentration at age 1 year (r = 0.30). The delay in the onset of the childhood component of growth and the association with age at start of treatment and initial L-thyroxine dose indicate that thyroid hormones during the first months of life are essential for normal onset of the childhood component of growth, which otherwise is assumed to be growth hormone-dependent.