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The most commonly instrument used to measure quality of life in patients with addictive behaviors is the WHOQOL-BREF, developed by the World Health Organization. No studies have been found to explore the psychometric properties in Spanish clinical samples. This paper aims to study their reliability as well as the construct validity in a representative sample of patients and comparing the results to the data available for the general population in Spain. A sample was recruited comprised of 523 subjects who were undergoing treatment with methadone in Madrid and Extremadura. A confirmatory factor analysis was completed to test the theoretically proposed structure and then an optimized-parallel analysis was done to establish the most adequate number of components. The result offers solid values for internal consistency both as concerns the items and the scales. The theoretical tetra-dimensional structure is confirmed in the sample although serious arguments are also found for considering its one-dimensionality. The structural relationship between the four domains was studied. The WHOQOL-BREF proves to be a reliable and valid test for use on patients treated with methadone, providing a multi-dimensional measure of perceived quality of life that includes social and environmental factors of great importance in treating addiction problems.