Described the development and validation of the Functional Disability Inventory (FDI) for school-age children and adolescents. Results provide support for construct, concurrent, and predictive validity. FDI scores also demonstrated stability over a 3-month period in patients with a chronic condition, and the instrument was sensitive to changes in patient status subsequent to medical treatment. There was some evidence that gender played a role in disability, particularly in adolescence. The instrument may be used (a) in studying individual differences in pediatric disability, (b) in examining the relation of disability to psychosocial functioning in the child and other family members, or (c) as an outcome measure in assessing the impact of interventions on patient functioning.