This retrospective study included 23 necrotic immature permanent teeth treated for either short-term (treatment period <3 months) or long-term (treatment period >3 months) using conservative endodontic procedures with 2.5% NaOCl irrigations without instrumentation but with Ca(OH)(2) paste medication. For seven teeth treated short-term, the gutta-percha points were filled onto an artificial barrier of mineral trioxide aggregate (MTA). For 16 teeth treated long-term, the gutta-percha points, amalgam, or MTA were filled onto the Ca(OH)(2)-induced hard tissue barrier in the root canal. We found that all apical lesions showed complete regression in 3 to 21 (mean, 8) months after initial treatment. All necrotic immature permanent teeth achieved a nearly normal root development 10 to 29 (mean, 16) months after initial treatment. We conclude that immature permanent teeth with pulp necrosis and apical pathosis can still achieve continued root development after proper short-term or long-term regenerative endodontic treatment procedures.