AIM: The aim of this study was to evaluate facial asymmetry and the thickness of the masticatory muscles in young children with normal occlusion and functional posterior crossbite. METHODS: The sample comprised 72 children of both genders (64.71±7.04 months) in the primary and early mixed stage of dentition, divided into four groups: primary-normal occlusion (PriN; N=19), primary-crossbite (PriC; N=19), mixed-normal occlusion (MixN; N=27), and mixed-crossbite (MixC; N=16). The thickness of the masseter and anterior portion of the temporalis muscle at rest and during maximal clenching were assessed by ultrasonography. Facial morphology and asymmetry were evaluated by standardized front-view photographs, in which the following measurements were recorded: anterior face height (AFH), bizygomatic facial width (BFW), angle of the eye (AE) and angle of the mouth (AM) (interpupillary and commissure planes in relation to mid-sagittal plane, respectively). RESULTS: The results showed that muscle thickness did not differ significantly between the sides of the dental arches in all groups (paired t-test). Only the groups with normal occlusion presented significant positive correlation between AE and AM (Pearson's correlation test). In PriN, only body weight was significantly related to masseter thickness; in MixN, facial morphology contributed significantly to masseter thickness at rest and maximal clenching, while the covariates weight, height and age did not relate to muscle thickness (stepwise backward multiple regression). CONCLUSION: In the studied sample, children with crossbite presented greater facial asymmetry than those with normal occlusion, and a greater masseter thickness was related to larger faces in the mixed dentition.