This study compares the responsiveness of three instruments of functional status: two disease-specific questionnaires (Oswestry and Roland Disability Questionnaires), and a patient-specific method (severity of the main complaint). We compared changes over time of functional status instruments with pain rated on a visual analog scale. Two strategies for evaluating the responsiveness in terms of sensitivity to change and specificity to change were used: effect size statistics and receiver-operating characteristic method. We chose global perceived effect as external criterion. A cohort of 81 patients with non-specific low back pain for at least 6 weeks assessed these measures before and after 5 weeks of treatment. According to the external criterion 38 patients improved. The results of both strategies were the same. All instruments were able to discriminate between improvement and non-improvement. The effect size statistics of the instruments were higher in the improved group than in the non-improved group. For each instrument the receiver-operating characteristic curves showed some discriminative ability. The curves for the Roland Questionnaire and pain were closer to the upper left than the curves for the other instruments. The sensitivity to change of the rating of Oswestry Questionnaire was lower than that of the other instruments. The main complaint was not very specific to change. The two strategies for evaluating the responsiveness were very useful and appeared to complement each other.