Objective To detect the effects of an apex locator-Root ZX, according to comparison of the results between Root ZX and X-ray. Methods Selecting 98 teeth, 186 root canals, using the Root ZX to measure the canal length, then preparing them, taking X-rays with the main gutta pertscha to compare the results. Results Their concordance is 88.7%, but there were no significant differences between vital pulp and dead pulp, single rooted tooth and multirooted tooth. Conclusions Root ZX can be used to measure the cana...