The authors compare the data from the psychological assessment of a cohort of 191 French speaking hypothyroid children detected by neonatal screening: 99 were followed in Quebec and 92 in Lille, according to two independent programs. Three age levels were selected: 12-18 months, 5 and 7 years. Data were similar and, in both studies, results of the later ages were slightly lower than the norms of the tests or results obtained in control groups. In both populations correlations were found between some results and two factors related to hypothyroidism: delayed bone surface development and thyroxinemia at the time of diagnosis. On the other hand, etiology and time of treatment had no repercussions on any group results.