The electromyogram(EMG) of lateral pterygoid muscle (LP) in 43 patients with LP dysfunction was measured before, when and after the use of occlusal splint. The postural and clenching activity of superior LP decreased dramatically when patients were wearing the occlusal splint. Normal EMG was observed in patients with normal arthrograra when the symptoms and signs were eliminated or reduced by occlusal splint. The EMG of dysfunctional LP in some patients with anteriorly displaced disk recovered to that with normal function as the incoordinated condyle -disk rektion was corrected. However, LP dysfunction was still present in most patients with irreversible anterior disk displacement and disk perforation after splint therapy.