The natural history of congenital hypothyroidism has changed drastically since the institution of newborn screening in North America, Europe, and Japan. Before screening was available, diagnosis of this condition was often delayed. The IQ of 65% of all patients with congenital hypothyroidism was below 85, and 19% were profoundly retarded (IQ less than 15). In two large screening programs in which long-term IQ data are available, no child has tested below the IQ of 74. Furthermore, a comparison of affected children treated within the first month of life with matched controls demonstrated no significant difference in IQ scores between the two groups. Widespread screening has lead to the evaluation of over 7 million infants annually. This represents over 1200 fewer children with subnormal intelligence, and approximately 360 children annually who are saved from the effects of profound mental retardation. There is little doubt that newborn screening is one of the greatest advances in diagnosis and treatment of endocrine disease in the newborn period to occur in the past 20 years. However, even experienced screening programs are not perfect, so the practitioner must remain alert to the possibility of undiagnosed hypothyroidism presenting in infancy.