Follow-up was obtained for at least 4 years in 52 children (40 F, 12 M) who presented with congenital hypothyroidism, detected by TSH screening at 5 days of age and persisting at the 3th-5th postnatal week. Eight had athyreosis, 33 an ectopic thyroid gland and 11 an anatomically normal thyroid. Treatment began at day 29 +/- 12 (1 SD) with I-T4 solution, 5-7 micrograms/kg/day and was followed in order to obtain immediately T4 and FT4 blood levels in the high normal range, with normal TSH at least from age 6 months. Mental development was evaluated at age 0.5, 1 and 2 years with the Brunet-Lézine scale, and at 4 years with the Termann-Merill test. Thirty of them were reevaluated at age 7 years with the WISC-R test. The results of these tests were all in the normal range, with individual differences, the range being 85 to 145 at 4 years and 95 to 133 at 7 years, not different from those obtained in age-matched controls and in the normal siblings of the patients at 4. The WISC-R test at 7 years did not show differences between verbal and performance levels. Its results were correlated with those of the TM test performed at age 4 years in the same 30 patients: r = 0.75, p less than 0.01. School level at 7 years was normal for age in these 30 hypothyroid children.(ABSTRACT TRUNCATED AT 250 WORDS)