External apical root resorption (EARR) is frequently associated with orthodontic treatment, however, the amount of root resorption is rarely of clinical significance. Severe EARR does, however, constitute a clinically significant problem. In contrast to the vast majority of the literature pertaining to EARR, the focus of this research project was restricted to only severe cases of EARR. The purpose of this study was to test whether frequently cited treatment-related variables played a significant role in the development of severe EARR. The difference between minor and severe EARR seems unlikely to be associated with the treatment that is received, but is instead inextricably linked to the genetic makeup of the patient.