With few exceptions, most cases of external apical root resorption have no evident etiology, particularly when root blunting from orthodontic treatment has been ruled out. This study determined the frequency of apical root resorption in the permanent dentition of patients who had not been treated orthodontically. Associations between the occurrence of external apical root resorption and three measures of oral health-number of missing teeth, periodontal probing depths, and alveolar crestal bone heights-were also examined. Between 7% and 10% of the 306 patients exhibited obvious apical resorption. Strong statistical associations were found between the occurrence of external apical root resorption and loss of teeth, increased periodontal probing depths, and reduced crestal bone heights; ie, root resorption was significantly more common in teeth with compromised support. Loss of stability from adjacent teeth, increased use of fewer remaining teeth, and loss of the root's anchorage in the bone are significant predictors of external apical root resorption.