OBJECTIVE To determine the prevalence and severity of impacted permanent mandibular second molars (MdM2s) in the general population of Taiwan as well as to evaluate treatment outcomes with an innovative method for impacted MdM2s. METHODS Dental records of 21,580 patients were retrospectively screened in this study. The impactions of MdM2s were registered. The chosen study population consisted of 96 patients with 125 impacted MdM2s who received orthodontic treatment in our clinics. Standardized periapical radiographs were taken to measure the angles and depths of the impacted MdM2s. In addition, the initial uprighting periods of the impacted MdM2s were recorded. RESULTS The prevalence of MdM2 impaction in the Taiwanese population was 0.65%. Most of the angles and depths of the MdM2s were 31-60° and 9-12 mm, respectively. The mean initial uprighting period of the impacted MdM2 was 3.6 months. There were no significant differences between genders in the initial uprighting period based on patient ages, impacted angles, or impacted depths (P > .05). Correlation analysis revealed that the impacted depth was highly and positively correlated with the initial uprighting period (r > .75). CONCLUSIONS The prevalence of MdM2 impaction in this study was low but slightly higher than previous reports investigating whites. Although such impactions are rare, it is recommended to diagnose early for an optimal treatment time and reduction of complications. In addition, the effective molar uprighting appliance used in this study has displayed good treatment outcomes with natural improvement of the adjacent infrabony defects of the first molars.