Aim: To determine with a cross-sectional study the prevalence of orthodontic treatment need using the Dental Aesthetic Index (DAI). Subjects and Methods: Using a stratified sampling method in private and public schools in Isfahan, Iran, 748 subjects were examined and divided into three age groups: 11 to 14, 14 to 17, and 17 to 20 years (355 females and 393 males, 15.11 ± 2.23 years), including 20 subjects with history of orthodontic treatment. DAI scores were recorded in those without history of orthodontic treatment (n = 728, 340 females and 388 males). The proportions of subjects within sexes needing treatment, as defined by having DAI scores of at least 31, were compared using the chi-square test. The chi-square test was also used to test for any difference for DAI treatment categories in different age groups and also to test for sex dimorphism. Results: The mean DAI score was 26.14 (SD = 7.64) points (95% CI, 26.60 to 26.72). On the whole, 54.5% showed no need or slight need for treatment. In 23.6%, the need for treatment was elective. However, in 11.0%, treatment was highly desirable and 10.9% showed very severe malocclusions and treatment was mandatory. There was a significant difference between sexes with regard to DAI treatment need categories (chi-square = 10.10, df = 3, P < .05). There was no significant difference between different age groups with regard to DAI treatment categories (P > .05). Conclusion: According to our findings, 21.8% of the evaluated Iranian school children in our sample had a DAI score above or equal to 31 points, suggesting highly desirable or mandatory orthodontic treatment need. World J Orthod 2010;11:e125–e132.

Key words: DAI, orthodontic treatment need, 11- to 20-year-olds, Iran