OBJECTIVES: A rigorous translation methodology is essential in order to develop equivalent language versions of health-related quality of life questionnaires for use in clinical research and trials. Using differential item functioning (DIF) within the item response theory framework to augment traditional reliability analysis provides additional evidence to evaluate whether the translated versions are psychometrically equivalent. METHODS: The Functional Assessment of Chronic Illness Therapy (FACIT) Measurement System translation methodology consists of 2 forward translations, 1 reconciled version, a back-translation of the reconciled version, and 3 independent reviews by bilingual experts. The translated versions are then pretested with at least 15 patients in the respective native countries. In this presentation, we focus on five translated versions of the FACT-G (Hungarian, Japanese, Polish, Portuguese, and Russian) that were each tested with at least 40 patients and analyzed; the total sample was 316 oncology patients. The internal consistency of the 27-item core FACT-G was evaluated using Cronbach's alpha. Rasch analysis was used to determine the item difficulty hierarchy of each language version, and then DIF analysis was used to determine psychometric equivalence in relation to the English version (n=131). RESULTS: The 27 core FACT-G items included in every FACIT measure showed high reliability in these five language versions, with Cronbach's alpha ranging from .87 (Japanese) to .93 (Polish). Items exhibiting potential DIF and their possible causes were summarized. CONCLUSION: Reliability and DIF analyses confirm that the five translated versions are psychometrically valid and comparable. The FACIT translation methodology has been effective in adapting a North American questionnaire to vastly different cultural contexts and creating valid versions for cross-cultural comparisons. Further research and development of new language versions is needed, especially in non-Western European languages to extend QoL research possibilities internationally.