Several studies have been conducted on sibling psychosocial adaptation to cancer in a brother or sister, but little is known on how the long-term adaptation of siblings to the illness develops. The concept quality of life has primarily been applied in research on the effects of chronic illness on the affected patient, but has not yet been studied in siblings. Aims: To investigate the prevalence of self-reported psychosocial problems in siblings of pediatric cancer patients, 2 years after the onset of the illness. Measurements: Two Dutch quality of life questionnaires, the TACQOL and the DUCATQOL are used, as well as generic non-health-related measures of emotional and behaviour problems (CBCL-YSR) and anxiety (STAI-C). Participants: Participants were 103 siblings aged 7-18 years old. Fifty seven Siblings participated in a prospective and 46 in a retrospective study group. Results: Siblings aged 7-11 report lower overall quality of life than children in the norm group. No differences in mean scores were found on any of the other domains that were investigated. When the prevalence of problems was investigated, however, relatively more siblings compared to normative data had scores in the impaired group based on the 20th percentile norm. A relatively high number of siblings aged 7-11 reported impaired emotional (42%), social (34%) and total quality of life (47%) (DUCATQOL) and physical problems (26%) (TACQOL). Relatively many adolescent siblings (26%) reported significant internalising problems on the CBCL-YSR. Conclusions: Although acute emotional distress reactions seem to have normalised in most siblings as has been suggested in the literature, emotional distress of having a brother or sister with cancer may continue beyond diagnosis for a subgroup of children. Young siblings seem to be affected in their quality of life, whereas a subgroup of adolescent siblings experience clinically relevant internalising problems. The results support the use of quality of life measures for siblings. Predictors of long-term adaptation in siblings need to be investigated.