INTRODUCTION: The purpose of this study was to identify risk factors for mandibular third-molar impaction in adolescent orthodontic patients. METHODS: Radiographs made before (T1) and after (T2) treatment and at least 10 years postretention (T3) of 134 patients that allowed accurate diagnosis of impaction vs eruption of at least 1 mandibular third molar were evaluated. RESULTS: Univariate logistic regression analyses at T1 showed that the decision to extract premolars (P <.05), an increase of 1 mm in mesial molar movement (P <.05), and an increase of 1 mm in eruption space measured from second molar to Xi point (centroid of the ramus) (P <.01) reduced the risk of impaction by 63%, 22%, and 30%, respectively. An increase of 1 degrees in mandibular third-molar angulation relative to the occlusal plane increased the risk of impaction by 11% (P <.01). Multivariate logistic regression analyses at T1 showed that reduced eruption space (P <.01), signs of forward mandibular growth rotation (P <.01), and female sex (P <.05) increased the risk of impaction. However, association tests documented increased frequencies of extractions and more mesial molar movement in female patients with erupted third molars. Univariate logistic regression analyses at T2 showed that every millimeter of increase in eruption space measured from the second molar to the anterior border of the ramus and to Xi point reduced the risk of impaction by 29% and 22%, respectively (P <.01), whereas every degree of increase in the angle between the occlusal surface and the occlusal plane increased the risk of impaction by 11% (P <.01). Multivariate logistic regression analyses at T2 showed that increased eruption space reduced the risk of impaction (P <.05), whereas increased mesial angulation of the third-molar bud (P <.01) and signs of forward growth rotation (P <.05) increased the risk of impaction.