This study was designed to investigate the effects of the occlusal splint on parafunctional oral motor behavior (grinding and clenching) during sleep in patients with bruxism and craniomandibular disorders. The results revealed that the splint does not stop nocturnal bruxism. In 61% of the patients, wear facets on the splint were observed at every visit (2-week intervals) and in 39%, from time to time. The wear facets reappeared in the same location with the same pattern and were caused mainly by grinding. The extension of the facets showed that, during eccentric bruxism, the mandible moved laterally far beyond the edge-to-edge contact relationship of the canines.