The authors present comparative data on the extent, nature, and impact of diagnosed mental disorder among children under 18 years of age seen during 1975 in four organized health care settings. Between 3.3% and 10.1% of the children seen were diagnosed as having a mental disorder in the study year, representing an annual prevalence of between 2.2% and 8.2% of the "covered" child populations. Transient situational disturbances, behavior disorders, and special symptoms were the most common diagnoses; the more severe disorders (organic brain disease, schizophrenia, and affective and other psychoses) accounted for less than 4% of all diagnosed mental disorder. Patients with diagnosed mental disorder used non-mental-health services (except those of pediatricians) appreciably more often than did patients without such a diagnosis.