The aim of neonatal thyroid screening is prevention of mental retardation by early diagnosis and early institution of thyroid replacement therapy. With relatively simple and inexpensive means it should be possible to shift the distribution curve of developmental and intelligence quotients as it was found in hypothyroid patients before the screening towards that one of healthy children. The results of our collaborative study show that we are approaching this aim. However, this study also demonstrates, that risk factors and associated findings may have a considerable influence on mental development and therefore should not be neglected in such an investigation. In response to our inquiry of 1985 we received detailed data on mental outcome of nearly 1,000 individual patients with CH representing not less than 14% of all children with CH detected in Europe since the introduction of neonatal thyroid screening. This shows that in many screening centers, a large number of children have not only been diagnosed and treated, but also followed carefully with respect to their development.