The purpose of the study was to explore the construct validity of three versions of the Oswestry Disability Questionnaire for low back pain using Rasch analysis. The three versions of the ODQ share 9 items and differ on one other. About 100 patients with non-specific low back pain seeking physiotherapy treatment at hospital outpatient departments and physiotherapy private practices completed the 12 Oswestry items as part of a battery of questionnaires. Rasch analysis revealed that four items (Personal Care, Standing, Sex Life and Social Life) had disordered response thresholds and one item (Walking) showed differential item functioning by age. The 10 standard Oswestry items and a modified version in which Sex Life is replaced by Work/Housework showed adequate overall fit to the Rasch model (chi(2)P>.01). The third version, in which Sex Life is replaced by Changing Degree of Pain, did not fit the model (chi(2)P=.006) and the Changing Degree of Pain item was misfitting (residual 2.34, P=.007). These findings suggest that either of the first two of the three versions of this widely used low back pain outcome measure should be selected over the third. Users should also be aware that for some items the rating scale steps do not perform as intended.