In order to evaluate the relationship between the mandibular position and the muscle activity during bruxism, an originally-designed device for detecting tooth contacts in eccentric jaw positions in addition to the intercuspal position was applied to 7 subjects and electromyography was carried out stimultaneously. Nocturnal bruxing patterns such as lateral grinding, clenching in the intercuspal position or eccentric jaw position were clearly distinguished by using this system. In one of 3 subjects who were not aware of bruxism, both the duration and the frequency of the muscle activity were significantly higher than those of other subjects. This suggests that some persons with bruxism who are unaware of it actually grind or clench their teeth during sleep. Tooth contacts during sleep were classified into 6 patterns according to jaw position. In the IP pattern, both regular and irregular EMG patterns were observed without any lateral movement of the mandible. Furthermore, each subject had the mandibular position on the preference side during tooth contacts and the EMG activity of the anterior temporal muscle on the preference side during the IP pattern was higher than that on the opposite side. This system enabled detailed evaluation of bruxism.