The mechanisms of mental retardation with relative prevalence in a hospital referral experience are reported from the Developmental Evaluation Clinic, Children's Hospital, Boston, MA. Early alterations of embryonic development (including Down syndrome) account for 32%, unknown causes 30%, environmental problems (psychosocial deprivation, childhood psychosis) 18%, pregnancy and perinatal morbidity 11%, hereditary disorders 5%, and acquired childhood diseases 4%. This classification uses the timing of the putative noxious event. The patients with mental retardation were obtained from over 3000 children referred for general developmental assessment to a tertiary children's medical center. 
COMMENT. This issue of Pediatric Annals also includes articles concerning community services for children with mental retardation and special needs adoption agencies.