Optimal mouth positioning for magnetic resonance imaging of the TMJ is controversial. This study evaluates the closed mouth position and partial-open mouth position for diagnostic accuracy of anterior displacement of the disk. Sixty-two joints were considered abnormal in at least one position. In the positive joints, the closed mouth position sequences were all abnormal and diagnostic, while 34% of partial-open mouth position sequences were falsely normal. It was found that they closed mouth position provides higher diagnostic accuracy for disk displacement, although optimal examinations include imaging in both positions.