Objective: To study the bilateral differences in the thickness of the masseter muscles in untreated individuals with lateral crossbite, as well as in subjects with successfully treated functional lateral crossbite, at least three years after the end of treatment. Materials and Methods: Three groups of growing individuals were studied: (1) untreated group: 38 individuals with unilateral crossbite, (2) control group: 224 subjects without transversal malocclusions, and (3) treated group: 18 individuals with functional lateral crossbite, treated with a quad helix, and at least three years out of retention with all permanent teeth erupted. The thickness of the masseter muscle was measured bilaterally with ultrasonography, using a real time scanner with a 7.5 MHz linear array transducer. Results: In the untreated group, the thickness of the masseter muscle on the crossbite side was statistically significantly thinner than the one on the normal side (P ϭ .025). No statistically significant differences were found in the thickness of the masseter muscle between the left and the right side in the control group. In the treated group, no statistically significant differences were found in the thickness of the masseter muscle between the former crossbite side and the normal one. Conclusion:The masseter muscle in untreated individuals with unilateral crossbite is thinner in the crossbite side when compared to the non-crossbite side possibly due to asymmetric activity of the masticatory muscles. Such an asymmetry in thickness of the masseter muscle could not be detected some years after the successful correction of the crossbite.