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Objectives: Bolton ratio is used as a method for diagnosis of tooth size discrepancy. Tooth size discrepancy has an important role in the stability of results of final orthodontic treatment. The aim of this study was to determine and compare Bolton ratio in normal occlusion and different malocclusion groups of Iranian population., Materials and Methods: This descriptive-analytic study comprises 240 orthodontic patient casts (for every malocclusion group: class I, class II division 1, class II division 2, class III, 30 males and 30 females) which have fully erupted and complete permanent dentitions from first molar to first molar, and also 60 persons (30 male-30 female) with normal occlusion. The mesiodistal tooth sizes were measured using digital caliper two times with a 4-week interval., Results: The average mesiodistal size of all teeth in the malocclusion group was significantly bigger than in the normal occlusion group. Although there was no relationship between anterior ratio and gender or malocclusion groups, but a significant relationship between total ratio in Class II division 1 and normal occlusion was seen. Furthermore, a significant difference between anterior and total ratios in the present study and original Bolton's values was found., Conclusions: In the normal occlusion group, the anterior and total ratios were bigger than those in Bolton's original study. Also, "class II division 1" showed the lowest values of Bolton ratio among the other normal occlusion and malocclusion groups. Therefore, the original Bolton's values should be used conservatively in Iranian population., (Copyright © 2019 Elsevier Masson SAS. All rights reserved.)