Objective: The purpose of this study was to retrospectively assess the success of endodontic treatment that had been guided by audiometric (electronic) measurement. Method and materials: The lengths of 66 infected root canals that demonstrated periapical pathosis were accurately measured by the Sono-Explorer before root canal obturation. The results over time intervals of 1 month to 20 years were evaluated on the basis of radiographic examinations. Results: The rate of successful treatment was 90.4% for short-filled root canals, 94.5% for flush-filled root canals, and a low 50.0% for long-filled root canals. The rate of successful endodontic therapy was 87.8% for restorations that did not exceed the apical foramen but reached the apical constriction and 95.3% if cases in which the apical radiolucencies were disappearing were included as successes. If cases of unintentional long-filling (overextension) were excluded, the success rate was as high as 98.4%. Conclusion: The poor performance of overfilled root canals indicates that practitioners should not overextend these restorations. Use of the Sono-Explorer aided successful treatment of infected root canals.