We performed a longitudinal study of a cohort of 74 children with congenital hypothyroidism (CH) detected by neonatal screening (Buenos Aires Province, Argentina) up to the age of 3 years old, in order to study linear growth and the relationship with the severity of CH at diagnosis. The mean age at diagnosis and the start of the treatment was 16.9 +/- 5.2 days. The patients were divided into group 1--severe CH (pretreatment T4 level <4 microg/dl) (n = 47)--and group 2--less severe CH (pretreatment T4 level > or = 4 microg/dl) (n = 27). Patients with CH treated early showed a sexually dimorphic pattern of growth: girls tended to be longer than boys at all ages. Boys showed some delay of growth during the first year. No difference was found in linear growth between the two groups (more/less severe CH). Height was normal in both sexes at the age of 3 years old.