OBJECTIVES: Fibromyalgia's complex symptomatology means that several instruments and questionnaires for assessing Quality of Life are used to collect a wide variety of information. Six questionnaires (Multidimensional Fatigue Inventory MFI, Multiple Ability Self-report Questionnaire MASQ, State-Trait Anxiety Inventory STAI, Fibromyalgia Impact Questionnaire FIQ, Beck Depression Inventory-II BDI-II, and Patient Global Impression of Change PGIC) were validated into twelve European languages in order to use them in international studies, METHODS: The process, carried out in collaboration with the questionnaire authors, comprised a forward translation by two translators who were native speakers of the target language, a backward translation by a translator who was a native speaker of the questionnaire source language, a review of the version obtained by an expert clinician and a comprehension test involving five healthy subjects from the country, RESULTS: The MFI, MASQ and STAI were validated in the target languages. The FIQ, BDI and PGIC in the eight, four and one missing languages. Irrespective of the questionnaire and dimensions studied, the same issues were encountered in their validation. 1) Issues were encountered in translation in a narrow sense. For example, the word "rug" was translated as "carpet"; 2) Literal translation was possible but largely culturally irrelevant. For example, the expression "to walk several blocks" was translated as "to walk more than one kilometer"; 3) Tenses needed to be reformulated or changed for idiomatic reasons. For example, the present perfect does not exist in German, and so the present simple was used in the first version. The imperfect was eventually used with adverbs such as "lately". CONCLUSIONS: Linguistic validation carried out using a rigorous and recognized method ensures that the concepts assessed are equivalent in twelve languages and allows their use in large scale studies measuring quality of life in fibromyalgia patients.