Determining the working length is of paramount importance for instrumentation, obturation and the efficiency of endodontic treatment. The radiographic method is the one most used by dentists to obtain this length; and in this method, the radiographic apex is of great importance, as it is the vital reference for obtaining the working length (WL). However, the exit of the apical foramen is the ideal anatomical landmark sought to delimit the endodontic procedure. However, these two entities do not always coincide. The aim of this study was to determine the location of the apical foramen of the distal root of mandibular first molars, comparing it with the radiographic apex. To this end, an in vitro study was carried out in which 30 mandibular first molars were selected, in which files were inserted up to the exit of the foramen and these were then radiographed. These radiographs were then subjected to measurement software. Measurements were taken from the radiographic crown-apex and crown-apical foramen.