Tosun G, Erdemir A, Eldeniz AU, Sermet U, Sener Y. Accuracy of two electronic apex locators in primary teeth with and without apical resorption: a laboratory study. International Endodontic Journal. Aim To evaluate the accuracy of the Root ZX (J Morita Corp., Kyoto, Japan) and Tri Auto ZX (J Morita Corp.) devices for measuring root canal length in primary teeth with and without apical resorption. Methodology Thirty-four extracted human primary molar teeth with resorption and 19 primary teeth without resorption were collected. After endodontic access preparation, the actual lengths of the teeth were determined. The teeth were then embedded in an alginate model to determine the electronic working length measurement. Statistical evaluation was completed using Student's t-tests. Results For the Root ZX, there was no significant difference between those teeth with root resorption and those without. However, for the Tri Auto ZX, there was a significant difference in the electronic measurements between those teeth with root resorption and those without (P < 0.05). Significant differences were found amongst the measurements of the two apex locators (P < 0.05). For root canals with resorption, the respective accuracy rates (within ±0.5 mm) of Root ZX and Tri Auto ZX were 83.33% and 89.47%; within ±1 mm, the Root ZX and Tri Auto ZX demonstrated 98.95% and 100% accuracy, respectively. For root canals with no resorption, the percentage of measurements within ±0.5 mm of the apical construction was 89.28% for the Root ZX and 80.35% for the Tri Auto ZX. The accuracy within ±1 mm of the Root ZX and the Tri Auto ZX was 98.22% and 100%, respectively. Conclusion Within the limitations of this laboratory study, the presence of resorption affected the performance of the Tri Auto ZX more than the Root ZX. © 2008 International Endodontic Journal.