In order to assess the influence of age at onset of treatment on subsequent growth, height, weight, head circumference (HC) and bone age as estimated by Greulich-Pyle and TW2-RUS methods, 100 children with congenital hypothyroidism (CH) were studied before and during adequate treatment up to 5 years of age. The patients were divided into five groups according to age at the start of treatment: Group 1: < 2 months (n = 26); Group 2: 2-3 months (n = 13); Group 3: 3-6 months (n = 21); Group 4: 6-12 months (n = 20); Group 5: 12-24 months (n = 20). Before treatment, groups 1 and 2 differed significantly from the others in height (p < 0.001). With hormone therapy, catch-up growth was observed in groups 3 to 5, but at age 5 years no differences were found between groups. In all groups, height at 5 years of age correlated significantly with children's midparental height (p < 0.002). Bone age was initially retarded in groups 3 to 5, but approximated the chronological age by age 5 years. Initially, HC was less affected than height and remained relatively larger up to age 5 years in all groups. These findings show that thyroid hormone replacement in CH as late as 24 months corrects the short stature and delayed bone age by age 5 years.