COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIORObjective: The Chalder Fatigue Questionnaire (CFQ) is an instrument used to measure physical and mental fatigue. We translated and adapted the questionnaire and tested its reliability and validity in a Brazilian primary care setting. Method: A pilot study with 204 consecutive primary care attenders in SÃO Paulo, Brazil, verified the internal consistency and factor structure of the questionnaire. After some modifications through a rigorous translation, back-translation, and cross-cultural adaptation procedure, a validation study was conducted with 304 attenders, who also completed the fatigue section of the Revised Clinical Interview Schedule (CIS-R). Results: The internal consistency of the Brazilian CFQ slightly improved from the pilot to the validation study: Cronbach's alpha from .86 to .88. The two-factor structure (physical and mental fatigue) also improved. According to the receiver operating curve analysis with the fatigue section of the CIS-R as the standard criterion, 3/4 was chosen as the cutoff for Brazilian primary care (sensitivity 69.1% and specificity 79.4%). Conclusion: The Brazilian CFQ had good reliability and validity. The cutoff was determined as 3/4 and the factor structure of the English CFQ was closely reproduced. (c) 2007 Elsevier Inc. All rights reserved