The Functional Disability Inventory (FDI; Walker LS, Greene JW. The functional disability inventory: measuring a neglected dimension of child health status. J Pediatr Psychol 1991;16:39-58) assesses activity limitations in children and adolescents with a variety of pediatric conditions. This study evaluated the psychometric properties of the FDI in pediatric pain patients. Participants included 596 patients with chronic abdominal pain, ages 8-17, and a subset of their parents (n = 151) who completed the FDI and measures of pain, limitations in school activities, and somatic and depressive symptoms at a clinic visit. Test-retest reliability was high at 2 weeks (child report, .74; parent-report, .64) and moderate at 3 months (child report, .48; parent report, .39). Internal consistency reliability was excellent, ranging from .86 to .91. Validity was supported by significant correlations of child- and parent-report FDI scores with measures of school-related disability, pain, and somatic symptoms. Study results add to a growing body of empirical literature supporting the reliability and validity of the FDI for functional assessment of pediatric patients with chronic pain.