The accuracy of an electronic device in locating the working length of root canals, without the use of radiographs, was compared with lengths determined by digital-tactile sense. Although the concept of the Sono-Explorer (an electronic device for measuring the length of root canals) is good, the predictability of accuracy of its intended purposes was less reliable than that with digital-tactile sense. Electronic instruments have been introduced for use in endodontics to measure the length of root canals and, hence, decrease the number of radiographs taken. This investigation was designed to evaluate the validity, predictability of accuracy, and value of such a device. The objectives were to determine the working length of root canals and to detect accessory or lateral canals electronically. Four investigators collectively tested 50 teeth with the electronic device and 50 teeth comparatively with digital-tactile sense. The results were verified with wire-grid radiology. Although the concept is good, the predictability of accuracy of its intended purposes was less reliable than that with digital-tactile sense. On the basis of the reported findings, the electronic device should not replace radiographic control in endodontic therapy as a measuring device. Further research and modifications are indicated. The accuracy of an electronic device in locating the working length of root canals, without the use of radiographs, was compared with lengths determined by digital-tactile sense. Although the concept of the Sono-Explorer (an electronic device for measuring the length of root canals) is good, the predictability of accuracy of its intended purposes was less reliable than that with digital-tactile sense. Electronic instruments have been introduced for use in endodontics to measure the length of root canals and, hence, decrease the number of radiographs taken. This investigation was designed to evaluate the validity, predictability of accuracy, and value of such a device. The objectives were to determine the working length of root canals and to detect accessory or lateral canals electronically. Four investigators collectively tested 50 teeth with the electronic device and 50 teeth comparatively with digital-tactile sense. The results were verified with wire-grid radiology. Although the concept is good, the predictability of accuracy of its intended purposes was less reliable than that with digital-tactile sense. On the basis of the reported findings, the electronic device should not replace radiographic control in endodontic therapy as a measuring device. Further research and modifications are indicated.