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Summary: Aim: To evaluate left and right masseter and anterior temporalis muscle activity in patients before and after orthognathic surgery. Patients: Nineteen patients were enrolled, 9 males and 10 females, aged 17–34 years. Four patients were suffering from a prognathic syndrome (skeletal class II with mandibular retrusion) and were candidates for surgical correction involving a mandibular Bilateral Sagittal Split Osteotomy (BSSO), whereas the other 15 patients showed a progenic syndrome (skeletal class III with mandibular protrusion) and were selected for bimaxillary surgery with maxillary advancement and mandibular retrusion. Methods: Electromyographic examinations were carried out on all subjects presurgically and 6–8 months postoperatively. To verify the neuromuscular equilibrium, the electromyographic activities of both the right and left masseter and anterior temporalis muscles were registered and analysed calculating: percentage overlapping coefficient (POC, index of the symmetric distribution of the muscular activity determined by the occlusion) and torque coefficient (TC, index of presence of mandibular torque). Results: After surgery, a trend in the improvement of POC and TC indices was found, with a reduced intragroup variability. Conclusion: The electromyographic evaluation allowed the impact of occlusion on neuromuscular equilibrium to be quantified, and showed that improvements gained by surgical intervention are primarily due to better occlusal stability and not to biomechanical advantages. [Copyright &y& Elsevier]