OBJECTIVE To evaluate and compare electromyographic activity of jaw muscles at rest and during maximal voluntary clenching in maximal intercuspation between growing individuals with unilateral posterior cross-bite without functional mandibular lateral shift during closure and matched individuals with normal occlusion. These findings may help elucidate the impact of this occlusal condition on jaw muscle function and the therapeutic implications. METHODS 30 growing children (10-12 years) with unilateral cross-bite without functional mandibular lateral shift during closure (cases) and 30 age- and sex-matched children with normal occlusion (controls) were consecutively selected applying highly restrictive criteria. Electromyographic activity coming from the areas of right and left anterior temporalis, posterior temporalis, masseter, and supra-hyoid muscles was evaluated at rest and during clenching. The asymmetry index was calculated for each muscle area to quantify the degree of asymmetry between sides. The masseter/anterior temporalis ratio during clenching was also recorded. RESULTS No significant differences were found between groups at rest. During clenching, activity of the cross-bite side masseter area was significantly lower in cases versus controls but no difference was found between cross-bite and non-cross-bite sides of this muscle. CONCLUSIONS Unilateral posterior cross-bite without functional mandibular lateral shift is associated with a lower electromyographic activity of masseter muscle area of the cross-bite side during maximal clenching in maximal intercuspation, compared to the children with normal occlusion. This result could suggest a functional adaptation that should be carefully considered. Long-term studies are needed to determine future repercussions and to evaluate the benefit of an early treatment.