Background: The agreement between children's self-reports and parent proxy-reports has not been established for function and quality-of-life measures for children with musculoskeletal diagnoses, including unilateral congenital below-the-elbow deficiency. Factors influencing parent-child agreement in this population have yet to be determined. Methods: Ten hospitals administered the Pediatric Outcomes Data Collection Instrument (PODCI) and the Pediatric Quality of Life Inventory (PedsQL) prospectively to children and adolescents with a unilateral congenital below-the-elbow deficiency in order to assess their function and quality of life. Two-thirds of the subjects wore a prosthesis. These children's and adolescents' self-reports were compared with their parents' proxy-reports for the PODCI (n = 179) and the PedsQL (n = 364). Results: Parents underestimated their children's/adolescents' self-report scores for the upper extremity physical function domain of the PODCI (p < 0.001) and overestimated the scores for comfort in the pain/comfort domain of the PODCI (p < 0.05). Parents also reported a lower social functioning score on the PedsQL than did the children and adolescents (p < 0.001). Greater agreement with regard to the social functioning domain of the PedsQL was observed between parents and children than between parents and adolescents (p < 0.05) and between parents and subjects who did not wear a prosthesis than between parents and subjects who wore a prosthesis (p < 0.01). Conclusions: Although the absolute differences are small, children with a unilateral congenital below-the-elbow deficiency report better upper-extremity function and quality of life than their parents perceive, but they may also be experiencing more pain. Factors influencing parent-child agreement on measures of quality of life include age and use of a prosthesis. Parents' reports of function may provide a helpful counterbalance to children's and adolescents' reports, but because quality of life is subjective by nature, the child's or adolescent's report is the gold standard. As a result of variability in agreement, PODCI and PedsQL parent reports cannot be considered true proxies for the self-reports of children or adolescents with unilateral congenital below-the-elbow deficiency or, possibly, of those with other musculoskeletal diagnoses.