[en] A mouth opener, an exercise device for the treatment of the disturbance of opening mouth, was utilized for a dynamic fast-scan magnetic resonance imaging (MRI) of the temporomandibular joint (TMJ). Thirteen patients suspected with internal derangements of the TMJ were examined with dynamic MRI and static spin echo imaging. Dynamic images clearly demonstrated anteriorly displaced disks and detailed features of its recapture. In the case of disk displacement without reduction, deformed disks resulting in restrictions of mouth opening were depicted. With the simple equipment available in the clinical use, the increased information from dynamic MR imaging warrants clinical application for the TMJ disease. (author)