Purpose: The KIDSCREEN project aims to develop a European cross-cultural generic self-administered Health-Related Quality of Life (HRQoL) instrument for children and adolescents. Proxy measures HRQoL should be a useful and practical alternative to assess children's HRQoL. The KIDSCREEN pilot study involved 3988 children and 2526 child-proxy pairs in seven European countries (Austria, Switzerland, Germany, Spain, France, United Kingdom, and the Netherlands). The proxy instrument is based on the model developed from the children and adolescents reports. The aim of this study is to assess the psychometric properties of the proxy measure in terms of reliability and construct and external validity. Methods: Confirmatory factor analysis (CFA) of the parent's data allows testing of the multidimensional structure of the proxy measure. Rasch analysis evaluates the scalability of each dimension. The mutltitrait-multimethod (MTMM) model assesses the trait validity through CFA. The agreement between children and proxies reports has been assessed using the Intraclass Correlation Coefficient (ICC). Results: CFA indicates that the children's model is adequate to the parents' data. Reliability is satisfactory for every dimension (CFI = .957). For every dimension, Rasch analysis indicates that items form a unidimensional continuum. MTMM results confirm the trait validity of the instrument. Higher agreement was found for the physical well being dimension (ICC=.52) and school/cognitive functioning (ICC=.52). Adolescents showed higher agreement than the children, and girls higher than boys. Children with physical chronic health conditions showed higher agreement for several domains: physical and psychological well-being, social support, and school environment. Conclusions: Exploring different facets of validity showed satisfactory results. This new instrument provides a promising measure to further assess the relationships between youth and proxy reports. © 2006 Society for Adolescent Medicine.