Study Design: Prospective, descriptive study. Objective: To determine the discriminant validity and intrarater and interrater reliability of measurements of temporomandibular joint (TMJ) range of motion (ROM). Background: Although ROM measurements are a routine part of the examination of patients with TMJ disorders, few studies have examined the validity of these measures for identifying pathology of the TMJ. Methods and Measures: Two groups of subjects were tested: 15 subjects (2 men, 13 women; mean age = 35.2 years) with a TMJ disorder and 15 subjects (3 men, 12 women; mean age = 42.9 years) without a TMJ disorder. Six motions were measured (opening, left excursion, right excursion, protrusion, overbite, and overjet) by 2 therapists. Results: Mouth opening was the only TMJ ROM measurement to discriminate between subjects with and without TMJ disorders (mean 36.2 2 6.4 versus 43.5 2 6.1 mm). The technical error of measurement of the measures varied from .2 to 2.5 mm. lntrarater reliability coefficients (ICC 3,l) varied from .70 to .99. Interrater reliability coefficients (ICC 2,k) varied from .90 to 1 .O. Conclusions: Although all TMJ ROM measurements tended to be reliable, only mouth opening was found to be valid in discriminating between patients with and without a TMJ disorder. ) Orthop Sports Phys Ther 2000;30:484492.