Translation and psychometric testing. To adapt the neck disability index (NDI) cross-culturally to Arabic language and to investigate the reliability and validity of the Arabic version of NDI in an Arabic-speaking sample with neck pain. Although largely used, no previous reports exist on the translation process or the testing of the psychometric properties of the Arabic version of the NDI. Cross-cultural adaptation of an outcome questionnaire. The English version of the NDI was translated into Arabic (NDI-Ar) and back-translated according to established guidelines. Sixty-five patients with neck pain completed the NDI -Ar twice during a 1-week period, to assess its test-retest reliability. Further psychometric testing was done by assessing internal consistency, construct validity (factor structure), and responsiveness. The internal consistency value (Cronbach α) for the NDI-Ar was 0.89. The test-retest reliability (intraclass correlation coefficient) was excellent at 0.96 (95% confidence interval from 0.93 to 0.97). There was a significant correlation (r = 0.92, P < 0.05) between the scores obtained from the first administration of the NDI-Ar and the second administration. Factor analysis demonstrated a 2-factor structure, explaining 67.58% of total variance. The analysis of responsiveness was calculated with an unpaired t test after 1 week of treatment and demonstrating a statically significant difference between stable and improved patients (P < 0.05). The Spearman correlation coefficient (rS = 0.81; P = 0.000) revealed strong relation between the change in score in the NDI-Ar and global rating of change. No ceiling or floor effects were detected in the NDI-Ar. The Arabic version of the NDI has a 2-factor 10-item structure and is a reliable, valid, and responsive tool that can be used to assess neck pain in Arabic-speaking patients with neck pain. Therefore, it can be recommended for clinical and research purposes.