Mental disorders affect 18% to 20% of children and adolescents. The rate in children with chronic illness is probably higher. This study of chronically ill children addresses the discrepancy between parent and child reports of child psychiatric disorders and the extent to which pediatricians agree with reports by children and parents regarding such problems. Eighty-three subjects, aged 9 to 18 (mean = 12.6), were recruited; they had the following diagnoses: cystic fibrosis, diabetes, inflammatory bowel disease, and cancer. Subjects and one parent were interviewed separately, using the Diagnostic Interview Schedule for Children (DISC-2.1). The subject's physician completed a questionnaire asking about the presence of a range of mental disorders. Forty-one (49%) subjects reached threshold criteria for a psychiatric diagnosis, using both parent and child as informants. Psychiatric disorders were identified in only 22 subjects (54%) by the child and in 28 (68%) by parent alone. Thus, reliance on one informant resulted in failure to identify one third to one half of psychiatric disorders. Physicians' ratings agreed significantly with children's reports but not with parental reports, suggesting that physicians are sensitive to children's concerns but may underestimate the value and importance of parents' reports. Clinical and research evaluations of chronically ill children, as well as clinician identification of mental health problems, will be influenced by the choice of informant.