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Crosscultural adaptation and psychometric measurements of the Functional Rating Index (FRI).To develop and investigate the reliability and validity of the Persian version of the FRI when applied to patients with low back pain (LBP).The FRI is a self-report questionnaire designed to assess patient's perception of function and pain. Despite the use of this outcome measure in clinical trials in Iran, it was not translated and culturally adapted.The FRI was translated into Persian language and crossculturally adapted. The Persian FRI was administered to 100 patients with LBP (male patients/female patients = 22/78). A numerical rating scale, the Roland-Morris Disability Questionnaire, the Oswestry Disability Questionnaire, and the Quebec Back Pain Disability Scale were also recorded. The test-retest reliability (time interval = 7 days) was assessed in 50 patients.There were no missing responses and floor or ceiling effects. The examination of discriminative validity showed that the questionnaire discriminated clearly between patients and healthy participants (P < 0.001). The concurrent criterion validity was confirmed by the Spearman rank correlation between the Persian FRI and the numerical rating scale (0.73, P < 0.0001 for test; and 0.77 for retest, P < 0.0001). Evidence for construct validity was found with a significant Pearson correlation between the FRI and the Roland-Morris Disability Questionnaire (r = 0.61; P < 0.0001), the Oswestry Disability Questionnaire (r = 0.75; P < 0.0001), or Quebec Back Pain Disability Scale (r = 0.76; P < 0.0001). Internal consistency reliability estimates (Cronbach α) for the Persian FRI were high, with 0.89 for test and 0.92 for retest. The test-retest reliability for the total score was excellent with an intraclass correlation coefficient (agreement) (ICCagreement) of 0.81 (P < 0.0001).The Persian version of the FRI seems to have an excellent reliability and validity when applied to patients with LBP.