Abstract Objectives To answer the clinical research question: in patients with myofascial pain, are there any differences in the surface electromyography (sEMG) activity of muscles of the painful and nonpainful sides that can be detected by commercially available devices? Methods The study sample (N = 39; 64%F, mean age 35.7 ± 15 years) consisted of patients seeking for temporomandibular disorders Temporomandibular Disorders (TMD) treatment and meeting Research Diagnostic Criteria for TMD (RDC/TMD) diagnosis of myofascial pain, with pain referred only in muscles on one side. They underwent sEMG of jaw muscles to record levels of standardized sEMG activity at rest, as well as during maximum clenching on teeth for the four investigated muscles, viz., bilateral masseter and temporalis. The existence of differences between sEMG values of muscles of the painful and nonpainful sides during the standardization test (i.e., clenching on cotton rolls) at rest and during clenching on teeth was assessed. Results At the study population level, differences between the sEMG values of muscles of the painful and nonpainful sides were not significant in any conditions, viz., either at rest or during clenching tasks. At the individual level, the difference between the sEMG activity of painful and nonpainful sides was very variable. Conclusions The above findings were not supportive of the existence of any detectable difference in sEMG activity between jaw muscles of the painful and nonpainful sides in patients with unilateral myofascial pain. Centrally mediated mechanism for pain adaptation may explain these findings, and the role of sEMG as a diagnostic tool for muscle pain needs to be carefully reconceptualized.