OBJECTIVES: The multiple sclerosis impact scale (MSIS-29) developed in UK English is a reliable, valid, and sensitive 29-item measure of the physical and psychological impact of MS on patients’ day-to-day life. For an international study, the MSIS-29 had to be translated into 29 languages for 19 countries. A thorough methodology was required to ensure conceptual equivalence and cultural relevance across languages. METHODS: A standardized methodology was conducted by specialists in each target country: 1) definition of original concepts in collaboration with the developer; 2) two forward translations by native target-language speakers; 3) comparison and reconciliation of the translations; 4) back translation by a native English speaker; 5) comparison of original and back translation; 6) comprehension test on 5 patients, native target-language speakers and suffering from multiple sclerosis and; 7) international harmonization. RESULTS: Two main challenges emerged. On the conceptual level, the English original made extensive use of synonyms to cover one concept which was not possible in most target languages and appropriate solutions had to be found. On the linguistic level, translating the medical terms present in the original proved challenging, as it was necessary to identify appropriate, non-technical idiomatic formulations in the translations to facilitate comprehension. Examples of these and other challenges and their solutions will be described in the presentation. CONCLUSIONS: According to the method used, the 29 language versions of the MSIS-29 were conceptually equivalent and culturally relevant. The translations may now be used in international studies to measure the physical and psychological impact of multiple sclerosis on patients’ day-to-day lives and facilitate the comparison and pooling of data. However, psychometric evaluation is required to determine stability of measurement performance across countries and cultures.