All authors agree that posterior crossbite is a malocclusion that affects mandibular growth and may lead to skeletal asymmetry but there are few data on which age these modifications are easily quantifiable. For this study, the researchers used x-ray records of 217 children between 6 and 9 years of age, in the mixed dentition stage and with unilateral posterior crossbite. All the horizontal variables were traced and evaluated by the principal researcher, using the tpsDig version 2 computer program. Subsequently, a descriptive and statistical analysis was carried out, using the SPSS 17.0 for Windows program. After analysing the vertical mandibular traces on the x-rays, the researchers found, in all cases, quantifiable differences between the crossbite side and the non-crossbite side. The differences between horizontal variables were statistically significant (p<0.005) for the entire sample (H3-H4), in the group of boys (H3-H4) and in the 7-year old age group (H1-H2 and H3-H4). Differences were observed in the size of the horizontal measures between the crossbite side and the non-crossbite side. Some of these differences were significant as a function of the sex and age of the study sample.