Aim: To compare in vivo the accuracy of five electronic apex locators (EALs) in determining the working length, set at a target reference length of 1.0 mm short of the radiographic apex. Methodology: Clinically healthy adult patients with a total of 100 teeth (232 canals) participated in this investigation. Informed written consent was obtained from each patient under a study protocol approved by an ethical committee from the Ministry of Health. Electronic working length (EWL) was determined by the apex locators, which were divided, for simplicity, into two groups with an equal number of canals (116 canals for each group). Group 1 consisted of the Root ZX II, the Apex D.S.P., and the Elements Diagnostic Unit and Apex Locator; group 2 consisted of the NaviRoot and the SiroEndo Pocket. All tested EALs were used according to their manufacturer's instructions. Radiographic working length (RWL) was determined by the operator prior to the EWL and was set at the general and universal agreement of 1.0 mm short of the radiographic apex. Verifications of the RWL measurements were blindly and randomly analysed by two independent evaluators (endodontists): IE1 and IE2. Finally, the results of the two groups were compared (intra-group comparison) using a paired sample t test. The level of significance was set at P 0.05. Results: Matching measurements between the apex locators and RWL, IE1 and IE2 were as follows: for Root ZX II, 104 canals (89.66%), 97 (83.62%) and 92 (79.31%), respectively; for Apex D.S.P., 80 canals (68.97%), 79 (68.10%) and 80 (68.97%), respectively; for Elements Apex Locator, 88 (75.86%), 87 (75%) and 85 (73.28%), respectively; for NaviRoot, 85 (73.28%), 84 (72.41%) and 84 (72.41%), respectively; and for SiroEndo Pocket, 81 (69.83%), 81 (69.83%) and 79 (68.10%), respectively. There were no statistically significant differences between IE1 and IE2, IE1 and RWL, and IE2 and RWL in both groups (P > 0.05). No statistically significant differences were found between the following variables: Root ZX II and RWL; IE1 and IE2; Apex D.S.P. and IE2; Elements Apex Locator and RWL, IE1 and IE2; and NaviRoot and RWL, IE1 and IE2 (P > 0.05). There was a statistically significant difference between the following variables: Apex D.S.P. and RWL and IE1; and SiroEndo Pocket and RWL, IE1 and IE2 (P 0.05). Conclusion: Within the limitations of this study, the results showed that all tested EALs achieved an acceptable determination of the root canal working length within ± 0.5 mm from the target reference point in a range of 75% to 95% of cases. Keywords: electronic apex locators, electronic working length, endodontics, radiographic working length