We examined length, height and weight from birth to six years of age and head circumference during the first two years in 89 children with congenital hypothyroidism (CH). The patients were divided in two groups: children diagnosed by clinical criteria during the first year of life (group A) and children detected by neonatal screening (group B). Group A showed a complete catch up growth for height and weight 10 months after the beginning of the replacement therapy; to the contrary, group B did not show any difference for height and weight compared to normal standards. Head circumference, evaluated only in group B, was significantly higher in comparison with normal standards. When etiology of CH was taken into consideration, children with athyreosis showed a significantly lower length at birth and at three months of age and their growths curves normalized after institution of replacement therapy. In conclusion our data suggest a direct relationship between severity and duration of hormone deficiency and growth retardation and confirm that replacement therapy started within the first year of live in CH patients clinically diagnosed allows a catch up growth.