BACKGROUND:Prevalence of pathological flatfoot has not been well defined in the literature. The objective of this study was to establish the prevalence of pathological flatfoot in school-age children and investigate the factors associated with this deformity. METHODS:A total of 667 children (327 girls and 340 boys) were recruited for this cross-sectional study with multi-stage clustered random sampling. The diagnosis of flatfoot was based on clinical observations and measurements using a suggested systematic protocol. Pathological flatfoot in this study did not merely screen for structural flatfoot, it included symptomatic conditions as well as musculoskeletal risk factors predisposing a progressive flatfoot in children. RESULTS:Prevalence of pathological flatfoot was 10.3% in children aged 7-14 years, but this prevalence decreased with age. There was no difference in prevalence of pathological flatfoot between the genders. Children with high BMI were more likely to have pathological flatfoot. Heel valgus (r=0.1; p=0.01) and dorsiflexion range (r=0.14; p<0.001) were both significantly associated with the presence of pathological flatfoot in children; but with only small correlations. CONCLUSIONS:Pathological flatfoot was a prevalent condition in school-age children in Iran. Forty six percent of the flatfeet observed in school-age children were classified as pathological.