Data sourcesCochrane Central Register of Controlled Trials, Medline, Embase and Scopus databases.Study selectionStudies that reported the precision of electronic apex locators (EALs) in locating the apical constriction (AC) in primary root canal treatment of human teeth compared with a histologic evaluation of the AC were considered.Data extraction and synthesisData were extracted and quality assessed independently by two reviewers.ResultsTen studies were included, reporting on 1105 EAL measurements. Seven studies were considered to be at high risk of bias and three at low risk. Four different EALs were evaluated; Root ZX (J Morita, Tokyo, Japan), Justy II (Hager & Werken GmbH & Co, Duisburg, Germany), Endy 5000 (Loser Co, Leverkusen, Germany) and Endox (Lysis Co, Milan, Italy). Three EALs, Root ZX, Justy II and Endy 5000 were more accurate than the Endox in determining the distance between the file tip and the apical constriction. Pulp status was only available for 194 (17.55%) of the measurements. The status of the pulp (vital or necrotic) had no significant effect on precision.ConclusionsThe precision of electronic working length measurement depends on the device used and the type of irrigation and is not influenced by the status of the pulp tissue.