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A longitudinal study using patient questionnaires was performed.To compare the discriminatory power and responsiveness of the Aberdeen Back Pain Scale (ABPS), the Roland Disability Questionnaire (RDQ), and the EuroQol in patients with low back pain.A number instruments specific to low back pain have not been compared for measurement properties. The EuroQol is a widely used generic instrument that has not been compared with specific instruments in patients with back pain.A questionnaire incorporating the Aberdeen Back Pain Scale, the Roland Disability Questionnaire, and the EuroQol was completed by patients taking part in a clinical trial of exercise treatments for back pain. Patients completed follow-up questionnaires at 6 weeks, 6 months, and 1 year. The discriminatory power of these instruments was assessed against variables relating to activity limitations, medication, and comorbidity. Responsiveness was assessed using standardized response means.The questionnaire was completed by 187 patients taking part in the clinical trial. The Aberdeen instrument was found to be the most powerful at discriminating between different groups of patients on variables relating to activity limitations, medication, and comorbidity. The specific instruments demonstrated good levels of responsiveness, with the Aberdeen instrument producing the largest standardized response means. The Aberdeen instrument was more responsive to the smaller changes experienced by the control group, but was less powerful than the Roland at measuring differences in the levels of change between the two groups of patients at two of the three follow-up assessments in the trial. The EuroQol demonstrated a moderate level of responsiveness.The two specific instruments are capable of greater levels of discrimination between groups of patients, and are more responsive over time than the generic EuroQol. The Aberdeen instrument performed most satisfactorily in relation to these criteria, but the Roland instrument was more sensitive to differences between the two groups in the clinical trial. The measurement properties of these two instruments reflect their origin: The Aberdeen instrument is based on clinical questions, whereas the Roland instrument is based on the generic Sickness Impact Profile. Instrument content should be carefullyconsidered when selecting instruments for applications, including clinical trials.