Background: Flat foot is a deformity in which patients lack standard arches in the soles of the foot. Flat foot improves with age. This study was designed to determine the prevalence of flexible flat foot among male school children in Tehran, Iran. Methods: A total of 1539 male school children were included in this study. Students were examined for the flatness of the sole of the foot as well as its degree and type. Flat foot was diagnosed on inspecting a poor formation of the arch. The severity of flatness was classified according to Denis criteria. Results: Among 1512 children whose foot condition was recorded, 80. 7% had normal/grade 1 sole, 16. 7% had 2 nd degree flat feet, and rd 2. 6% had 3 degree flat feet. Prevalence of genu varum was 16. 1% and prevalence of genu valgum was 2. 7% (P = 0. 504). The prevalence of flat foot decreased significantly with age,in the group of 11-year-old boys and younger, 23. 1% showed a grade 2 and grade 3 flat foot, whereas in the group of 12-year-old boys and older, only 13. 6% had a grade 2 and grade 3 flat foot (P < 0. 001). Conclusion: In this study, a significant relationship was found between weight gain and body mass index (BMI) with flat feet. There is a significant association between passing the age of 12 years and a decrease in flat foot. Also, the association between flat foot with weight and height of children was examined separately, and it was found that weight could significantly be a poor predictor of flat feet.