Parafunctional activity (toothgrinding, toothclenching and bruxism) is a common problem which may lead to masticatory muscle and temporomandibular joint pain, and may result from sleep arousal or disturbances. Sleep apnea is another common sleep disorder which results in disrupted sleep architecture and frequent arousals. Because sleep apnea leads to sleep arousals, and because sleep arousals are thought to result in increased parafunctional activity, we undertook the present study to determine the relationship between sleep apnea and parafunctional activity. We were also interested in assessing the effects of sleep posture on sleep disordered breathing and parafunctional activity. We prospectively studied 24 patients who were referred to the clinical sleep apnea laboratory for study. They underwent standard nocturnal polysomnographic examination; in addition, masticatory activity was measured with a masseter electromyogram. Patients slept in the supine and lateral decubitus positions. Nocturnal clenching was slightly higher in patients with sleep apnea than those without (12.2 vs 7.6 clenches/hr, p = 0.18), and there was a correlation between the clench index (CI) and apnea plus hypopnea index (A + HI) by linear regression (r = 0.49, p less than 0.05). There were significant falls in both the A + HI (64.4 +/- 28.8 vs 36.5 +/- 36.7, p = 0.02) and CI (12.5 +/- 12.1 vs 7.0 +/- 8.6, p = 0.04) in the lateral decubitus vs supine sleeping positions. We conclude that there is an association between obstructive sleep apnea and parafunctional activity, that sleep position affects the incidence of both sleep disordered breathing and parafunctional activity, and that analysis of apneas and hypopneas in both supine and lateral decubitus sleeping positions may be helpful.