PURPOSE: Fatigue is one of the most common symptoms associated with cancer and its treatment. To obtain a more precise and flexible measure of fatigue, the EORTC Quality of Life Group has developed a computerized adaptive test (CAT) measure of fatigue. This is part of an ongoing project developing a CAT version of the widely used EORTC QLQ-C30 questionnaire. METHODS: Based on the literature search and evaluations by experts and patients, 41 new fatigue items were developed (in addition to the three QLQ-C30 fatigue items). Psychometric properties of the items, including evaluations of dimensionality, fit to item response theory (IRT) model, and differential item functioning (DIF), were assessed in an international sample of cancer patients. RESULTS: Responses were obtained from 1,321 cancer patients coming from eight countries. Factor analysis showed that 37 of the items could be included in a unidimensional model (RMSEA = 0.098, TLI = 0.995, CFI = 0.920). Of the 37 items, two were deleted because of poor fit to the IRT model forming the basis for the CAT, and one because of DIF between cancer sites. CONCLUSIONS: We have established a 34-item fatigue bank allowing for more precise and flexible measurement of fatigue, while still being backward compatible with the QLQ-C30 fatigue scale.