To examine parent-professional agreement in proxy-reports of child quality of life (QoL) and the factors associated with low child QoL in children with cerebral palsy (CP) and associated intellectual impairment. Professional (teacher, therapist, or residential carer) and parent reports of QoL for 204 children (127 males, 77 females, mean age 10y 4mo [SD 1y 6mo]; range 8-12y) with CP and IQ less than or equal to 70 were obtained in 2004 to 2005 in nine European regions, using the KIDSCREEN questionnaire. Parent-professional agreement was studied using correlation and mean differences; multilevel logistic regression was used to determine factors influencing QoL reports and agreement. The mean parent-reported scores of child QoL were significantly higher than the professional reports in the Psychological well-being domain and significantly lower for Social support. The average frequency of disagreement (parent-professional difference greater than 0.5SD of scores) over all domains was 62%. High levels of stress in parenting negatively influenced parent reports of child QoL compared with professional reports, while child pain was associated with professionals rating lower than parents. Proxies do not always agree when reporting the QoL of children with severe disabilities. Parental well-being and child pain should be taken into account in the interpretation of QoL reports in such children. (Contains 3 tables and 1 figure.)