We evaluated the longitudinal response in 43 infants with congenital primary hypothyroidism during the first year of L-thyroxine therapy. Diagnosis was confirmed by serum thyroid hormone measurements by 4 weeks of age in 38 infants and between 40 and 80 days of age in the remainder. This group of infants was divided by radionuclide thyroid imaging into 34 infants with thyroid dysgenesis and nine with dyshormonogenesis. The group with thyroid dysgenesis was subdivided into 21 infants with athyreosis and 13 with residual thyroid tissue (11 ectopic and 2 hypoplastic glands). L-Thyroxine therapy, at an average dose of 10 to 14 micrograms/kg/day, was begun immediately after diagnosis, and serum concentration of total thyroxine, free thyroxine, triiodothyronine, reverse triiodothyronine, and thyroid-stimulating hormone were determined serially. Serum concentration of total and of free thyroxine became normal within 1 week of the start of therapy in all groups. Despite a similarly mild degree of hypothyroidism at diagnosis observed in infants with dyshormonogenesis or with ectopia or hypoplasia, those with dyshormonogenesis had a more sensitive response to initial thyroid hormone replacement than did patients with thyroid dysgenesis, as judged by L-thyroxine does and thyroid-stimulating hormone suppression. We conclude that the prompt restoration of clinical and biochemical euthyroidism during early infancy with doses of L-thyroxine between 10 and 14 micrograms/kg/day is a safe and effective method of therapy for children with congenital hypothyroidism.