Systematic screening for congenital hypothyroidism in the newborn was introduced some 15 years ago. The main objective was the prevention of mental retardation due to thyroid hormone deficiency during the early months of life. During the past decade screening programs have become routine throughout most of the industrialized world and many questions relating to implementation, organization and quality control of such programs have been largely resolved. Preliminary IQ and neurological data have indicated that screening and early treatment do, in fact, prevent mental retardation. However, a number of scientific questions related to congenital hypothyroidism remain unanswered and extensive research activities are ongoing in the field. The objective of the organizers of the Brussels workshop was to focus almost exclusively on these current research aspects of the screening programs. This workshop is the third international conference specifically devoted to neonatal thyroid screening. The first was held in La Malbaie in Quebec in the fall of 1979. That meeting was well organized and highly productive. Its proceedings constitute a bible in the field. After the Quebec meeting, we witnessed major and rapid advances in our understanding of neonatal thyroid physiology as well as screening methodology, organiza- tion, data management, the significance of an approach to false negative and false positive results, patient follow-up, and assessment of follow-up and treatment, and the psychoneurological evaluation of affected infants. Some of these aspects were further developed during a second highly pro- ductive international conference organized in Tokyo in 1982.