BACKGROUND This retrospective study was conducted to assess the eruption of third molars by using panoramic radiographs and to identify the variables associated with unsuccessful eruption. METHODS The subjects were 48 patients who had 128 permanent second molars extracted during or before orthodontic treatment. Their ages at extraction were 11 to 23 years. The position of the third molars was assessed from panoramic radiographs taken before second-molar extraction and after third-molar eruption. The median time of eruption was 3 to 4 years (interquartile range, 2 years). A successful final position was defined as eruption with proximal contact with the adjacent first molar and an angle between these 2 teeth of no more than 35 degrees. RESULTS A total of 96.2% of the maxillary and 66.2% of the mandibular third molars erupted in good positions. The maxillary third molars uprighted and successfully replaced the second molars. In the case of Nolla developmental stage > 8, the proximal contact could remain open. Most mandibular third molars uprighted and successfully replaced the second molars. Most unsuccessful eruptions of mandibular third molars were due to excessive mesial tilting or lack of proximal contact. Unsuccessful third-molar eruptions occurred in older patients who had higher Nolla developmental stages. CONCLUSIONS Maxillary third molars upright and acceptably replace maxillary second molars after extraction for orthodontic purposes. However, if the Nolla developmental stage is > 8, proximal contact could remain open. Most mandibular third molars also upright and acceptably replace the second molars after extraction. Unsuccessful third-molar eruption is most common in older patients with higher Nolla developmental stages. Most unsuccessful eruptions are due to excessive mesial tilting or lack of proximal contact.