Objective: Cell phones can release electromagnetic interference (EMI) and their usage is commonly forbidden in many hospitals to prevent potential interference with medical devices. The purpose of this clinical study was to assess the impact of three smartphones on electronic apex locator (EAL) working length (WL) determination. Materials and methods: Forty teeth (30 anterior; 10 posterior) with 60 canals were chosen for the study. The effect of three brands of smartphones, when present or absent and in proximity, on an EAL was investigated. The WL of teeth determined using an EAL, in the presence or absence of a Smartphone were compared with the digital radiography. The collected data were analysed using one-way ANOVA; the statistical significance was set at 0.05. Results: Ten canals showed a disparity in the WL determination in the presence of smartphones. There was a significant difference (P = 0.039 and P = 0.001) in the WL by the three methods in anterior and posterior teeth respectively. Conclusions: From this study, it can be concluded that smartphones can affect the WL determination when using an EAL. Smartphones should not be placed in close proximity to an EAL during use. Parole chiave: digital radiography, electronic apex locator, smartphones, working length