Autotransplantation of premolars with incomplete root development is a reliable treatment option for tooth replacement. Nevertheless, little is known about root development after transplantation of teeth. The aim of this study was to evaluate preoperative root development stage and radiographic width of the apex as predictors for root elongation after autotransplantation. A retrospective cohort study was implemented and composed of patients who underwent autotransplantation of at least 1 premolar. Using panoramic radiographs (orthopantomograms), calibrated measurements were taken to record the root development stage of the donor tooth, width of the apex, and root length of the transplanted tooth. Forty-five patients (58 transplants) were included. Mean follow-up time was 12months (range, 4 to 26months). Fifty-three transplanted teeth (91.4%) showed root elongation. Mean root elongation at end of follow-up was 1.9mm (range, 0.0 to 4.3mm). A wide-open apex before transplantation was significantly associated with root elongation after autotransplantation (P<.01). Root development stage (P=.82) did not show significant associations with root elongation. A wide-open apex (>2.0mm) is an important predictor of root elongation after autotransplantation of premolars. This might be a better predictor for root development than preoperative root development stage.