Little is known of the effects of an interocclusal appliance on the activity of craniocervical muscles in patients with temporomandibular disorder. The bilateral electromyographic activity of the frontalis muscle and sternocleidomastoid muscle were assessed in 10 patients with temporomandibular disorders and eight gender, age and dentally matched asymptomatic control subjects during rest and different clenching tasks with or without an interocclusal appliance. Clenching significantly increased both frontalis and sternocleidomastoid activity from rest. Clenching with an interocclusal appliance, as compared with clenching alone, also significantly reduced frontalis activity in both groups, but not for sternocleidomastoid. Increased electromyographic activity of these muscles may represent co-activation of the craniocervical musculature during clenching.