Over the past 10 years, a plethora of back-specific patient-orientated outcome measures have appeared in the literature. Standardisation has been advocated by an expert panel of researchers proposing a core set of instruments. Of the condition-specific questionnaires the Oswestry Disability Index (ODI) is recommended for use with low back pain (LBP) patients. To date, no Danish version of the ODI exists which has been cross-culturally adapted, validated and published in the peer-reviewed literature. A cross-cultural adaptation and validation of the ODI for the Danish language was carried out according to established guidelines: 233 patients [half of the patients were seen in the primary sector (PrS) and half in the secondary sector (SeS) of the Danish health care system] with LBP and/or leg pain completed a questionnaire booklet at baseline, 1 day or 1 week and 8 weeks follow-up. The booklet contained the Danish version of the ODI, along with the Roland Morris Disability Questionnaire, the LBP Rating Scale, the SF36 (physical function and bodily pain scales) and a global pain rating. For the ODI test-retest analysis (93 stable patients) resulted in an intraclass correlation coefficient of 0.91, a mean difference of 0.8 and 95% limits of agreements of - 11.5 to + 13. Thus, a worsening greater than 12 points and improvement greater than 13 points can be considered a "real" change above the measurement error. A substantial floor effect was found in PrS patients (14.1%). The ODI showed satisfactory cross-sectional discriminant validity when compared to the external measures. Concurrent validity of the ODI revealed: (a) a 10% and 21% lower ODI score compared to the disability and pain measures, respectively, (b) a poorer differentiation of patient disabilities and (c) an acceptable individual ODI score level compared to the external measures. Longitudinal external construct validity showed moderate correlations (range 0.56-0.78). We conclude that the Danish version of the ODI is both a valid and reliable outcome instrument in two LBP patient populations. The ODI is probably most appropriate for use in SeS patients.