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Background: The European Organisation for Research and Treatment of Cancer (EORTC) Quality of Life Group (QLG) has developed computerised adaptive tests (CATs) for the 14 functional and symptom domains of the EORTC QLQ-C30 quality of life questionnaire. This is expected to optimise measurement precision, relevance to patients and flexibility. Here, we present the first international validation of the EORTC CAT Core., Methods: A heterogeneous sample of 699 cancer patients scheduled for chemotherapy and/or radiotherapy was recruited across seven European countries. The EORTC CAT Core and all QLQ-C30 items were administered to participants before and after initiating treatment. Correlations between CAT and QLQ-C30 scores and floor/ceiling effects were calculated. Using several grouping variables, relative validity (cross-sectional known groups difference), responsiveness (changes over time) and relative sample size requirements of the CAT compared to the QLQ-C30 were estimated., Results: Correlations of the CAT and QLQ-C30 ranged from 0.81 to 0.93 across domains. The mean relative reduction in floor and ceiling effects using the CAT was 42% (range 3-99%). Analyses of known groups validity and responsiveness indicated that, across domains, mean sample size requirements for the CAT were 72% and 70%, respectively, of those using the QLQ-C30., Conclusions: The EORTC CAT Core measures the same domains as the QLQ-C30 with reduced floor/ceiling effects. The CAT generally facilitated the use of smaller samples (about 30% smaller on average) without loss of power compared to the QLQ-C30. Based on this study, the EORTC QLG will release the EORTC CAT Core for general use.