OBJECTIVES: Symptom burden is critical for assessing therapeutic efficacy in myelofibrosis, characterized by splenomegaly, abdominal and constitutional symptoms, often painful and debilitating (Mughal et al. 2014).  Developed from the widely used MPN-SAF, the 7-item MPN-SAF TSS was modified with wording changes and by adding one tiredness item to better meet regulatory needs and assess symptom severity in myelofibrosis to support treatment benefit endpoints in clinical research. Study objectives were to translate the mMPN-SAF TSS into 21 languages for 26 countries and to assess comprehension and conceptual equivalence with the English source version in order to expand its use in global myelofibrosis research. METHODS: The mMPN-SAF TSS was translated following ISPOR guidelines for linguistic validation of PRO measures (Wild et al., 2005) using the universal approach (Wild et al., 2009). For each non-English language, two forward translations by native translators, reconciliation of forward translations, back-translation by one English-speaker fluent in the target language, and final reconciliation by a native-speaking language coordinator were conducted. Harmonization (i.e., comparison between languages for consistent interpretation) was performed to ensure conceptual equivalence across languages.  Interviews were conducted with 4-5 native-speaking myelofibrosis patients for each language/country combination (N=142). Interview data were analyzed qualitatively to assess linguistic and cultural validity in each language and confirm conceptual equivalence. RESULTS: Mean age was 60 years (range 25-88); 51% were male. The translations were well understood and considered relevant with some patients raising minor issues during interviews.  Patient feedback resulted in wording changes to the Bulgarian, Czech, Danish, Dutch, German, Hebrew, Hungarian, Japanese, Korean, Malay, Russian, Spanish, Simplified and Traditional Chinese versions.   CONCLUSIONS: All translated versions of the mMPN-SAF TSS were confirmed to be conceptually equivalent and well understood in the 26 countries evaluated. These translations fill a significant need for patient-focused instruments in local languages to facilitate future myelofibrosis research worldwide.