In 1946 a random sample (N = 5,362) of all children born in England, Wales, and Scotland during 1 week in March were selected for a longitudinal study of health and development. Educational, clinical, and psychological information were obtained from this national cohort at repeated intervals. At age 15, all children who had experienced a chronic physical disorder were identified. Their clinical profiles were defined and the social, psychological, and educational data on this group (N = 528) were compared with the remaining physically healthy children (N = 4,196). A total prevalence rate of 111 chronically ill children per thousand under age 16 was found. This figure is of the same order as those reported for other studies of a similar kind. Most of the children (58%) suffered from a condition which primarily affected motor activities; 20% involved vision, speech or hearing, and 20% affected appearance. Most disorders were mildly disabling and over 60% were still present at age 15. The composition of this population was compared with that of the remaining healthy children with respect to sex, social class, family size, and parents' health. These comparisons provide a background for subsequent reports describing the psychological, educational, and social consequences of chronic illness during childhood.