The purpose of this in vitro study was to evaluate the accuracy of Root ZX apex locator to determine the working length in teeth with simulated apical root resorption. Fifty extracted, single-rooted, human teeth with mature apices were used in this study. An irregular cavity defect was drilled at the apex of each tooth simulating an apical root resorption. Three operators used the Root ZX to measure the working lengths, comparing the electronic readings with the direct visual measurements. The Root ZX was 62.7%, 94.0%, and 100.0% accurate to within 0.5 mm, 1 mm, and 1.5 mm of the direct visual measurements, respectively. Statistically significant differences were observed between operator A and B and A and C (p < 0.01), but no significant differences were detected between operator B and C (p > 0.01).