OBJECTIVE To determine the dimensional changes in the mandibular arch of untreated subjects during the late mixed dentition stage. METHODS The cross-sectional study was conducted over three years starting March 2013 at the College of Dentistry, King Saud University, Riyadh, Saudi Arabia, and comprised consecutive models taken annually of subjects seeking dental treatment. They were aged 9 years at baseline and dimensional changes were calculated at 9, 10 and 11 years of age. Measurements taken each year were overjet, overbite; inter-canine width, canine length, inter-first deciduous molar width, inter-first premolar width, inter-first permanent molar width, arch length, molar depth, incisor irregularity, and available anterior space. SPSS 16 was used for data analysis. RESULTS Of the 15 subjects, 10(66.6%) were girls and 5(33.3%) were boys. Of all the variables, the mean values of overbite (p=0.006) and inter-canine width (p=0.001) increased significantly over the period, while the mean irregularity index values decreased significantly (p<0.0001).. CONCLUSIONS An increase in overbite and inter-canine width, and a reduction in the irregularity index of teeth was detected during the mixed dentition phase.