Background: Myofascial pain syndrome (MPS) is a common cause of neck pain, which is a global public health problem. Because MPS does not present morphological changes within lesioned muscles, there are no imaging diagnostic criteria for this condition. In this study, we evaluate elasticity changes in upper trapezius muscles most frequently involved in cervical MPS using real-time ultrasound shear-wave elastography, and we examine their potential diagnostic value. Methods: We consecutively enrolled 109 right posterior neck pain patients for this prospective study. Of these, 51 were diagnosed with MPS and 58 with non-MPS in the right side of their neck. Among MPS patients, 19 fell into the 1-3 range (mild pain) for pain scores on the visual analog scale (VAS), 25 fell into the 4-6 range (moderate pain), and 7 into the 7-10 range (severe pain). MPS was diagnosed by two independent clinicians using the diagnostic criteria proposed by Simons et al. Using real-time ultrasound shear-wave elastography, we measured right trapezius mean shear-wave velocity (SWVmean). The midpoint of the line between the foramen magnum and the end of the right acromion served as measuring point. Regions of interest were scaled to span 0-8.0 m/s. Results: Trapezius SWVmean was significantly higher in MPS patients compared with non-MPS patients (P<0.001). Stratified analysis of MPS patients according to pain severity revealed similar trapezius SWVmean between mild pain and non-MPS patients (P=0.324), however SWVmean was higher in moderate and severe pain MPS patients compared with non-MPS patients (P<0.001). The area under the curve (AUC) value for upper trapezius SWVmean in MPS patients was 0.791 (95% CI: 0.703-0.863). Corresponding sensitivity and specificity values were 86.27% (95% CI: 73.7-94.3%) and 62.07% (95% CI: 48.4-74.5%). Stratified analysis of MPS patients by pain severity produced the following AUC values for trapezius SWVmean in MPS patients with mild, moderate, and severe pain: 0.578 (95% CI: 0.460-0.690), 0.899 (95% CI: 0.814-0.955), and 0.983 (95% CI: 0.914-0.999), respectively. Conclusions: Elasticity changes and increased stiffness in the trapezius occur in cervical MPS patients with moderate and severe pain. The SWVmean parameter reflecting trapezius muscle elasticity may be valuable for successful screening of cervical MPS, especially in patients with moderate and severe pain.