El propósito de este estudio fue establecer si se debe modificar la longitud apical en dientes con periodontitis apical crónica, que presentan reabsorción apical del cemento, de dentina radicular o de dentina intraconducto, o de la combinación de estas, y que pueden afectar la unión cemento dentinaria se tomaron 18 dientes humanos que se dividieron en 10 casos experimentales y 8 controles. Se sometieron a un proceso de desmineralización y fijación para obtener placas histológicas que se observaron al microscopio de luz. En el grupo control no hubo inflamación apical en el 100% y, el 50% presentó reabsorción superficial que afectó solamente al cemento, el 12.5% mostró reabsorción intraconducto. Se encontró que los dientes experimentales presentaron inflamación apical el 60% y reabsorción externa inflamatoria el 70%, esta se presentó tanto en el cemento como en la dentina. Por los análisis histológicos debemos pensar que la longitud de la endodoncia no debe tener variación aun si el diente tiene periodontitis apical crónica o no tiene. ABSTRACT The purpose of this study was to establish if it necessary to alter the working length of endodontically treated teeth which present chronic apical periodontitis with associated apical root resorption of dentin or cementum , internal dentinal resorption or a combination of either one these and which affect the CDJ. Eighteen teeth which required extraction for orthodontic treatment, were selected and divided into an experimental group of 10 teeth which presented chronic or acute apical periodontitis and a control group of 8 which had a normal clinical appearance. An evaluation under light microscopy was done of each tooth after subjecting them to a demineralization and fixation process. Analysis of the control group revealed that no apical inflammation was evident in any of the teeth, 50% presented superficial resorption which affected the cementum and 12.5% exhibited internal resorption. On the other hand, 60% of teeth evaluated in the experimental group evidenced apical root resorption, out of which 70% was present in both dentin and cementum. Results suggest therefore that the extension of endodontic treatment should not be altered regardless of the presence of apical periodontitis