This paper reports the first part of a three-part study of developmental defects of tooth enamel in a pediatric hospital population. The dental records of 8411 children who were discharged from the Department of Dentistry at the Royal Children's Hospital, Melbourne, Australia, between 1960 and 1987 were divided into an experimental group of 7518 patients comprising 25 groups of medical conditions, and a control group of 893 children who had dental disorders only. The aim of the study was to investigate the prevalence of hypoplastic and severe-opacity developmental defects of tooth enamel (DDE), in children and adolescents with major medical disorders, and to compare the prevalence with that in the control group of normal children. The prevalence figures obtained for the different medical conditions in this study agreed generally with those of other recent investigators. The high prevalence of defects found in Rubella Embryopathy children (81.8%) and in children with Prematurity alone (56.5%) is surprising, whereas the prevalence of 27.9% defects in Clefts of Lip and Palate and 26.4% defects in Clefts of Lip and Alveolus are probably well below the true prevalence. The control group prevalence was 9.3%, which is higher than in some other studies of 'normal' children. A pediatric hospital is a most useful source of fully documented medical and dental histories for the investigation of possible relationships between medical disorders and developmental defects of tooth enamel. The control group prevalence was 9.3%, which is higher than in some other studies of normal children. A pediatric hospital is a most useful source of fully documented medical and dental histories for the investigation of possible relationships between medical disorders and developmental defects of tooth enamel.