Pre- and posttreatment radiographs of 2050 orthodontically treated patients were examined and apical root resorptions of more than 2 mm were identified. 30 patients (211 teeth) were affected. Treatment characteristics and objectives of this group (with severe apical root resorption) were compared to a treated group of 30 patients (with no resorption) to determine clinical risk factors. Extrusion, but especially jiggling and a long treatment period were found to be significantly more frequent in the group exhibiting resorption than in the control group. Pathogenesis of treatment-induced root resorption is discussed in respect to histological results.