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Cross-cultural validation of the Neck Disability Index (NDI) and Neck Pain and Disability Scale (NPDS).To translate and test the psychometric properties of Thai versions of the NDI and NPDS questionnaires.The NDI and NPDS are questionnaires commonly used to measure neck pain and disability. The original versions of the NDI and NPDS have been translated into many languages but there are no validated Thai versions.Translation and cross-cultural adaptation of the original English versions of the NDI and NPDS were performed according to published guidelines. A total of 185 patients with neck pain participated in the study and were asked to complete the Neck Disability Index-Thai version (NDI-TH), Neck Pain Disability Scale-Thai version (NPDS-TH), and a visual analogue scale (VAS; 0–10). Psychometric evaluation included factor analysis, internal consistency, test-retest reliability, agreement, and convergent validity. Test-retest reliability for the NDI-TH was examined in 20 patients and in 17 for the NPDS-TH. Patients completed the questionnaires twice with an interval of 48 hours.Factor analysis revealed a single-factor solution for the NDI-TH and three factor solution for the NPDS-TH. Cronbach α values for the NDI-TH and NPDS-TH were excellent (0.85 and 0.96, respectively). The three NPDS-TH subscales ranged from 0.90 to 0.94. Good to excellent test-retest reliability was demonstrated for all measurements (Intraclass Correlation Coefficient range = 0.74–0.91, P < 0.001). The NDI-TH, NPDS-TH, and NPDS-TH subscales were moderately correlated with the VAS (range, r = 0.61–0.76, P < 0.001). The minimal detectable change was 16.1 points for the NDI-TH (scale range = 0–100) and 21.8 points for NPDS-TH (scale range = 0–100).The NDI-TH and NPDS-TH seem to be reliable instruments to measure functional limitation and disability in Thai patients with neck pain. However, further research is still needed to determine responsiveness and validity of these questionnaires.