The central nervous system is probably the most vulnerable organ in the fetus and the new-born. Since mental deficiency is the most common manifestation of abnormality of the central nervous system occurring in the neonatal period, it would appear reasonable that a review of the etiologic diagnoses for a large number of persons with mental deficiency might throw light on the relative importance of the various factors that may play an injurious role in fetal and neonatal life. The purpose of this paper is to present a critical analysis of the etiologic diagnoses for 1,330 patients with mental defects. MATERIAL AND METHODS The patients studied represent all those admitted to the Southbury Training School, Southbury, Conn., since its inception, in October 1940, for whom a reasonably reliable diagnosis could be made on the basis of the available history and clinical study. The majority of the patients, roughly 70 per cent,