Aim: Electromagnetic interference (EMI) emitted by smartphones can disrupt the functioning of some medical devices. The aim of the present in vivo study was to determine the effect of two smartphones (IPhone 5S and Samsung Galaxy S5) on the reliability of two electronic apex locators (EALs) (Novapex and Root ZX II). Materials and methods: Twelve patients were enrolled in the present study. Thirty-one root canals from sixteen teeth in need of root canal treatment were selected, after clinical and radiographic examination. The effect of smartphones (IPhone 5S and Samsung Galaxy S5) on both tested EALs (Novapex and Root ZX II) was determined under two different conditions: no smartphone in the operatory room (control group); and smartphones with Wi-Fi and Bluetooth setting activated and placed in physical contact with the EAL to maximise the chance of detecting EMI. The EWL was measured three times per tooth under each condition. To evaluate the reliability of EALs when used near smartphones, all measurements were submitted to ANOVA and Tukey's test with a significance level of 5%. Results: It was possible to determine EWL under all the experimental conditions. No significant differences (P > 0.05) were found for EWL measurements in the presence or absence of smartphones for the two tested EALs. A linear correlation between the two different tested EALs in the presence or absence of smartphones was also observed. Conclusion: It can be concluded that mobile phones used in the present study did not affect accuracy of EWL measurements in vivo. Keywords: cell phones, electronic apex locator, endodontics, Odontometry