OBJECTIVE: To investigate usefulness of surface electromyography (sEMG) for the pain evaluation in the patients with temporomandibular myofascial pain dysfunction syndrome (TM-MPDS). METHOD: Twenty unilateral TM-MPDS patients (10 males, 10 females, duration of disease 6.6+/-5.6 months, age 28.5+/-3.0 years) and twenty healthy controls (10 males, 10 females, age 30.1+/-6.2 years) were recruited for this study. The patients were treated with trigger point injection therapy and cryotherapy once a week, and ultrasound thermotherapy four times a week for 2 weeks. Active electrodes were attached on temporalis and masseter muscles with reference electrode on chin and ground electrode on forehead. All of subjects were instructed to clench teeth for 5 seconds after full relaxation. For temporalis and masseter muscles, the amplitudes of clenching-EMG activities were obtained and the mean value of five biting cycles was calculated on both sides. sEMG, visual analog scale (VAS) and asymmetry index (AI) were assessed before and 2 weeks after treatment. RESULTS: For temporalis and masseter muscles, the amplitudes of affected side-EMG activity significantly decreased, and the AI of the patient group significantly increased compared with those of the control group (p<0.05, p< 0.05). The AI of the patient group significantly decreased after treatment (p<0.05, p<0.05). Higher AI in masseter muscle before treatment was significantly correlated with lower VAS after treatment (r=-0.487, p<0.05). CONCLUSION: It is suggested that sEMG would be useful for diagnosing and evaluating the pain in TM-MPDS.