To explore the relationships among dysfunction of the two heads of the lateral pterygoid muscle, temporomandibular joint (TMJ) internal derangement, and the mechanism of TMJ sounds, a comparative study was undertaken with electromyography of the two heads of the muscle and arthrography in 38 patients with temporomandibular joint disturbance syndrome sounds. Results suggested that the electromyograph shows various characteristics of dysfunction, when compared with the electromyograph of normal adults, that are significantly relevant to temporomandibular joint internal derangement, especially anterior disk displacement with reduction and without reduction. The mechanism of the sounds was further explained on the basis of the observation of this study. Summing up the data, the authors propose that the dysfunction of the superior head of the muscle is one cause of temporomandibular anterior disk displacement.