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Background: The prevalence of idiopathic planovalgus decreases with age among children and adolescents. Previous studies have provided averages of radiographic indices for different age groups but not information about the rate of spontaneous correction and the affecting factors. The aim of this study was to estimate the rate of spontaneous improvement of radiographic indices of idiopathic planovalgus through the application of a linear mixed model. Methods: We included patients with idiopathic planovalgus who were no older than the age of fifteen years, who had had two or more weight-bearing foot radiographs made, and who were followed for more than one year. The talonavicular coverage angle, the anteroposterior talus-first metatarsal angle, the calcaneal pitch angle, and the lateral talus-first metatarsal angle were measured on the radiographs. The rate of angular correction was adjusted by multiple factors with the use of a linear mixed model, with sex and laterality as the fixed effects and age and each subject as the random effects. Results: A total of 568 feet were included in this study, and a total of 3284 radiographs were measured. The talonavicular coverage angle was found to have decreased by 1.7° per year (p < 0.001); the anteroposterior talus-first metatarsal angle, by 2.1° per year (p < 0.001); and the lateral talus-first metatarsal angle, by 0.7° per year (p = 0.034). The spontaneous improvement of the calcaneal pitch with aging was not significant. Conclusions: The talonavicular coverage angle and the anteroposterior talus-first metatarsal angle on anteroposterior radiographs and the lateral talus-first metatarsal angle on lateral radiographs improved as patients with idiopathic planovalgus grew older. These findings can assist in the prediction of the radiographic improvement of idiopathic planovalgus. Level of Evidence: Prognostic Level IV. See Instructions for Authors for a complete description of levels of evidence. [ABSTRACT FROM AUTHOR]