Evaluation of changes in muscle thickness, bite force and facial asymmetry during early treatment of functional posterior crossbite.OBJECTIVE:  To determine morphological and functional effects on masticatory system of early treatment of functional posterior crossbite in young children. STUDY DESIGN:  23 children were divided into two groups: deciduous (DecG, n = 11) and early mixed dentition (MixG, n = 12), which received slow maxillary expansion. Maximal bite force, ultrasonographic masticatory muscle thickness and facial asymmetry were evaluated in three stages: before the start of treatment (s1), after three months of retention (s2), and after three months of observation (s3). The results were analyzed by Mann-Whitney U-test, correlation test, repeated measures ANOVA and backward stepwise multiple regression. RESULTS:  Bite force and temporalis thickness increased from s1 to s2 and s3 in both groups (p < 0.05). Body mass index (BMI) increased significantly from s1 to s3 only in the MixG, but the masseter thickness did not differ among the stages. The correlation between the angle of the eye and the angle of the mouth in relation to the mid-sagital plane increased from s1 to s3. Masticatory muscle thickness contributed significantly to bite force magnitude in all stages, whereas age and BMl showed no significant contribution to its variation. CONCLUSION:  Bite force and temporalis muscle thickness increased significantly in children after early treatment of functional crossbite.