A prospective study of the mental development of hypothyroid infants detected by the Quebec Network for Genetic Medicine began in January, 1976. The mean age at initiation of thyroid hormone therapy was 27 days. Forty-five hypothyroid infants and 37 normal control subjects were assessed at age 12 months with the Griffiths mental development test; 77 and 41, respectively, were assessed at age 18 months, and 59 and 40, respectively, at 36 months. There were no statistically significant differences in the various test scores between the two populations at age 12 months, but at age 18 and 36 months the hypothyroid infants had lower scores in hearing-speech performance scales and practical reasoning (36 months) which also decreased their global quotient. The mean scores were still above 100 and only nine were below 85. Further assessment of the influence of early therapy on mental development at age 6 years is needed before definitive statements can be made about the long-term mental development in these subjects.