Parafunctional activities are assumed to play an important role in temporomandibular disorders (TMD), but experimental data in support of this hypothesis are lacking. This study examined the role of parafunctional clench ing on various measures of TMD pain. Five subjects participated in daily 17-minute electromyogram biofeedback training session structured in three phases. Subjects were instructed to maintain temporalis and masseter muscle activity below 2 uV in the first (decrease) phase of training (10 sessions), above 10 uV in the second (increase) phase (1 to 8 sessions), and below 2 uV in the third (decrease) phase (10 to 15 sessions). Preliminary screening examinations showed that none of the subjects had TMD. Two subjects reported intolerable pain during increase training, and both were diagnosed with a TMD during this phase. No subject was diagnosed with TMD pain during either decrease training phase. The authors conclude that chronic, low-level parafunctional clenching may be a factor in the cause of TMD pain.