Introduction: This study evaluates the endodontic working-length measurements in teeth with large periapical lesions and persistent intracanal exudate by using preexisting cone-beam computed tomography (CBCT). It compares the measurements with clinical root canal lengths determined by using 2 electronic apex locators. Methods: All patients had undergone a CBCT scan independent of the present study and needed root canal treatment of at least 1 tooth visible in the field of view. Seventy-three teeth with single roots and canals were studied. An endodontist measured each root canal length with 2 different electronic apex locators. The measurements were repeated 3 times by using a digital caliper, and the mean was recorded. This mean was compared with the root canal length as measured on CBCT sections by an oral radiologist not involved in the endodontic treatment. The CBCT measurements' were taken twice for analysis of intrarater reliability. The data were statistically analyzed. Results: There were no significant measurement differences between the methods used (P > .05). Conclusions: In teeth with large periapical lesions and persistent intracanal exudate, measurement of the root canal length by using CBCT was as reliable as measurements that used apex locators.