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This study tested the hypotheses that (1) parafunctional clenching increases pain and can lead to a diagnosis of temporomandibular disorder (TMD) pain and (2) electromyographic (EMG) activity during parafunctional clenching is significantly and positively correlated with reports of pain. Fourteen individuals without TMD participated in 5 consecutive days of 20-min long EMG biofeedback training sessions of the left and right temporalis and masseter muscles. Subjects were randomly assigned to either a Decrease or Increase group and were instructed to maintain EMG activity below 2 μV or above 10 μV during training, respectively. Two Increase subjects and no Decrease subjects were diagnosed, by a blinded examiner, with TMD pain following training. Self-reported pain posttraining was significantly higher for the Increase group. Masseter EMG activity was strongly correlated with pain. Parafunctional clenching increases pain and can lead to a diagnosis of TMD in otherwise pain-free individuals. Pain reports are positively correlated with the activity of the masseter muscle.