Children, like adults and parents, depend on primary care physicians to identify, treat, or refer those with mental disorders. Mental health concerns are also germane to the growing number of chronically ill children in pediatric care. This paper focuses on: 1) the level of agreement between children and parents about the presence of a mental disorder and the impact of informant on case identification; and 2) the extent to which pediatricians agree with reports by chronically ill children and/or their parents regarding such problems. The study sample includes 112 children, ages 9 to 18, with cancer, cystic fibrosis, inflammatory bowel disease, and insulin-dependent diabetes and a control group of 35 healthy subjects. Subject and a parent were interviewed separately using a highly standardized, structured interview that generates DSM-III-R diagnoses by computer algorithms. Pediatricians completed a questionnaire asking about the presence of any mental disorders. Agreement between parent and child was poor for both groups. In the medically ill group, more cases were identified by the parent interview than by child interview alone for all types of disorders. In contrast, children in the comparison group more often reported symptoms sufficient for a diagnosis than did their parents. The difference in prevalence between the two groups was significant only for the parent-identified cases, and physicians were more likely to recognize child-identified disorders. The choice of informant(s) has clear implications for case identification and case recognition in both clinical care and research in pediatric settings.