OBJECTIVES: The aim was to perform a cross-cultural adaptation and validation of the Fatigue Severity Scale (FSS) for use in Brazilian patients with myopathy and who complains of precocious muscular fatigue. METHODS: The FSS presents nine items measured on a Likert scale ranging from 1 (completely disagree) to 7 (completely agree), where higher scores indicate higher level of fatigue. The process of cross-cultural adaptation included: two independent translations for Portuguese spoken in Brazil; the development of a consensual translated version; application in a pilot group (n=14) of patients with myopathy; evaluation by an expert committee for content validation; a back-translation by one bilingual translator whose native tongue was English, but who was fluent in Brazilian Portuguese. The two English versions (original and back translated) were analyzed by two of the authors and a final Brazilian version was obtained. Twenty one patients with muscular disease following at the outpatient clinic from a University Hospital answered the Brazilian version of the FSS, the visual analogue scale (VAS) and the Chalder fatigue questionnaire (CFQ). The following analyses were performed: exploratory factorial analysis; internal consistency (Cronbach’s alpha); construct validity through of the correlation with VAS and CFQ (physical and mental components). RESULTS: The FSS scale obtained in the process of cross cultural adaptation was comprehensible to individuals in the pilot population. The twenty one patients who participate in the validation process were aged 21 to 65 years. The exploratory factor analysis determined one factor, as the original version. Reliability analysis indicated satisfactory internal consistency (0.93). Construct validity of the FSS (total score) with VAS and CFQ demonstrated moderate correlations (0.60 and physical=0.56, respectively). The FSS didn´t correlate with mental component of the CFQ (0.31).  CONCLUSIONS: The FSS scale is an instrument reliable and valid to measure muscular fatigue in Brazilian patients with myopathy.