Purpose: The aim of the present study was to determine the prevalence of malocclusion among schoolgirls and to assess malocclusion using the Index of Orthodontic Treatment Need (IOTN). Materials and Methods: A total of 350 subjects were selected by a multistage randomised cluster sampling method. All of the selected subjects were schoolgirls aged 14 to 17 years. Malocclusion was determined using the IOTN. A chi-square test was performed to evaluate the data obtained. Results: The present study reported that 16.3% of subjects had no treatment need, 35.7% had a mild treatment need, 35.4% had a moderate treatment need and a further 5.2% to 7.4% had a severe to extreme treatment need, respectively. Therefore, the prevalence of malocclusion was found to be 83.7% among Tehranian schoolgirls of age 14 to 17 years. Conclusions: The prevalence of malocclusion is very high among Tehranian schoolgirls; therefore, greater care is required to reduce this prevalence. Keywords: epidemiology, IOTN, malocclusion, orthodontics, prevalence, Tehran