When quality of life questionnaires are used as measures of treatment outcomes, it is essential to know how well these can respond to clinical changes. The objective of this study is to examine the responsiveness of the Brazilian-Portuguese version of the Oswestry Disability Index (ODI-Brazil) in subjects with chronic low back pain submitted to a physical therapy program. Thirty subjects with chronic low back pain completed the ODI-Brazil questionnaire, along with an 11-point pain visual analogue scale (Pain VAS), and the Brazilian-Portuguese version of Roland-Morris disability questionnaire before and after the program. All patients also completed a global perception of change Likert scale in condition after the program. This scale was collapsed to produce a dichotomous variable outcome: improved and non-improved. Responsiveness was determined using effect size statistics and receiver operating characteristic curve (ROC curve), with best cut-point analysis. The best change score cut-off was identified when equally balanced sensitivity and specificity was found, as an expression of the minimum clinically important difference (MCID). After treatment, 19 patients considered themselves improved. Both the effect size (0.37) and the area under the ROC curve (0.73) for ODI-Brazil score in relation to global outcome after program indicated that the ODI-Brazil showed responsiveness. The ROC curve for ODI-Brazil was distributed at the upper corners of the diagonal line, indicating that the questionnaire presents discriminative ability. The best cut-off point for ODI-Brazil was approximately 4.45 points (63.2% sensitivity, 81.8% specificity). The Brazilian-Portuguese version of ODI has comparable responsiveness to other commonly used functional status measures and is appropriate for use in chronic low back pain patients receiving conservative care.