["The study's aim was to assess the internal reliability for the nine domains of the European Organisation for Research and Treatment of Cancer Quality of Life Questionnaire (EORTC QLQ-C30) to evaluate homogeneity across clinical studies and whether sample characteristics predict coefficient heterogeneity.", 'A systematic literature review was undertaken. Internal reliability was assessed against Cronbach α coefficient >0.70. Reliability generalization was undertaken using fixed- and random-effects models. A weighted least squares regression model was applied to determine whether baseline sample characteristics (language, percentage of women, sample size, sample means and standard deviations, and cancer type) predicted variation in α coefficients.', 'A total of 33 studies were identified. Eight domains demonstrated good internal reliability (unweighted/weighted by sample variance). One domain, Cognitive Functioning, consistently performed poorly. In terms of moderating variables, none of the sample characteristic variables explained sample variance for the Physical or Role Functioning domains. For the other domains, language, percentage of women, and sample means and variances accounted for some of the heterogeneity observed.'] Are most domains of the European Organisation for Research and Treatment of Cancer Quality of Life Questionnaire C30 reliable?