Study Design: Cross cultural adaptation and psychometric testing.Objective: To conduct the cross-cultural adaptation of the Quebec Back Pain Disability Scale (QBPDS) and investigate its reliability and validity in patients with chronic low back pain (CLBP).Summary of Background Data: The QBPDS is one of the most commonly used scales to evaluate functional incapacity resulting from low back pain. Although measuring disability is an important outcome in physiotherapy care there is no previous research relating to the cultural adaptation and psychometric testing of the QBPDS in the Portuguese speaking population.Methods: The questionnaire was first translated and back translated following the published guidelines. The Portuguese version of the QBPDS was then piloted in a Portuguese sample of 40 patients with CLBP. Psychometric properties were evaluated in a new sample of 132 patients with CLBP. Exploratory factor analysis was performed in order to confirm its unidimensionality. Reliability was evaluated through internal consistency and reproducibility, using Cronbach's alpha and intraclass correlation coefficient (ICC2,1), respectively. Construct validity was assessed with correlations between the QBPDS and the Roland Morris Disability Questionnaire (RMDQ), and Visual Analogue Pain Scale (VAS) for convergent validity, and pain localization for discriminative validity, using Spearman's correlation analysis and the Mann- Withney Test.Results: Exploratory factor analysis revealed the existence of one major factor that explains the 52.1% of the variance. One week test- retest reliability was 0.7 and internal consistency was 0.95. The QBPDS correlated strongly with the RMDQ (0.62, p< 0.001), moderately with pain (0.38, p<0.001), and shows capability to discriminate between patients with localized and referred pain (U = 1218, p<0.0005).Conclusion: The reliability and construct validity of the Portuguese version of the QBPDS is acceptable to assess functional status of Portuguese speaking patients with CLBP.