The aim of the present study was to evaluate the clinical value of using the lateral tibiocalcaneal (LTiC) angle, measured on a lateral radiograph made with the foot and ankle in maximal dorsiflexion, instead of the ankle dorsiflexion (ADF) angle, measured on physical examination, as a determining indicator of the need for percutaneous Achilles tenotomy after Ponseti serial cast treatment of idiopathic clubfoot.We identified patients with idiopathic clubfoot who had been treated at our institution between March 2006 and June 2012. After exclusion of patients with Diméglio grade-I clubfoot and those followed for less than two years, 125 idiopathic clubfeet were evaluated. The clubfeet were grouped according to their LTiC and ADF angles, after which we evaluated the clinical outcomes of each group and investigated the prognostic effects of each angle-based decision regarding whether to perform percutaneous Achilles tenotomy.Patients with a favorable LTiC angle (≤80°) showed no sagittal relapse even though none underwent percutaneous Achilles tenotomy. In contrast, patients with an unfavorable LTiC angle (>80°) had a high chance of sagittal relapse if percutaneous Achilles tenotomy had not been performed, even when they had a favorable ADF angle (≥15°). The LTiC-angle-based decision showed the most significant prognostic value in multivariate analyses of relapse-free and surgery-free survival rates.The LTiC angle measured on simple radiographs is a more objective and prognostic indicator of the need for percutaneous Achilles tenotomy, as compared with the ADF angle measured on physical examination, in patients with idiopathic clubfoot.Prognostic Level III. See Instructions for Authors for a complete description of levels of evidence.