The establishment of apical foramen patency and its maintenance during chemomechanical preparation has been considered an important step in the cleaning and disinfection of the apical part of the root canal. The purpose of this study was to evaluate the adaptation of endodontic files to the apical foramen of extracted teeth using scanning electron microscopy. Twenty mandibular first premolars were divided into two groups. In group 1, adaptation to the apical foramen was examined by taking note of the first file to bind in the canal when moved 1 mm beyond the foramen. In group 2, adaptation was evaluated using a file one size larger than that of group 1. Files were fixed in the canal and adaptation to the contour of the apical foramen was examined under a scanning electron microscope. The area occupied by the file in relation to the foramen area was also calculated. Findings demonstrated that no instrument, regardless of the group, adapted to the apical foramen. The foraminal area occupied by the instrument cross-section ranged from 7% to 54%. Lack of adaptation was due to the much larger diameter of the apical foramen and/or its irregular shape in relation to the patency file. Keywords: apical foramen, dental anatomy, endodontic treatment, patency files, root canal preparation