The aim of this study was to analyze the agreement between self and proxy reports of quality of life (QoL) in people with intellectual disabilities and to examine the factors which contribute to these differences. The study was conducted across six international centres in a sample of 614 adults with intellectual disabilities as well as two different samples of proxies (N = 874) including both professional carers and relatives. QoL was assessed with the disability version of the WHOQOL-BREF. In both proxy samples results show a significant moderate association between the persons' QoL-assessment and the assessment of their proxies in all of the five QoL domains. There were significant mean differences in most items which varied in their direction. In general, people with disabilities rated their QoL higher than their proxies except for two items from the physical domain. The factors which most significantly contributed to these differences were proxy knowledge of the person as well as the WHODAS-disability score. Cross-cultural differences in the magnitude of concordance were observed. As a conclusion, the study shows a good agreement between the person-proxy QoL-assessments, and that the factors associated with disagreement are limited in this study. Future studies should elaborate the stability of concordance over time. (Contains 6 tables.)