Purpose: There is always a risk of invasion of the implant into adjacent tooth. The purpose of this study is to evaluate the clinical outcome of the implant and the invaded adjacent natural tooth when the implant directly invades the adjacent tooth or comes close to it. Materials and Methods: The present study was conducted on patients of Seoul National University Bundang Hospital who had received implant placements that implant directly invaded an adjacent tooth or came close to the root of the adjacent tooth (≤1 mm). Among a total of 35 patients (19 males, 16 females, mean age 61.2±11.1), 39 implants were investigated. The implants which invaded natural teeth were classified into three types (direct invasion, contact with the root of the tooth, and being close to the root of the tooth with less than or equal to 1 mm distance) using a periapical radiograph. The survival rate of the implants and clinical prognosis of the invaded teeth were investigated. Results: The most invaded natural teeth were canine and premolars. Among the invaded teeth, 39 teeth that had a proper medical record were investigated. Direct invasion had occurred in 12 cases, contact with the root of the tooth in 13 cases, and being close to the root of the tooth (≤1 mm distance) in 14 cases. A total of two implants failed and removed, and the average survival rate was 94.9%. As for the state of invaded natural teeth, 27 teeth (69.2%) functioned in a sound and vital state. Three invaded teeth (7.7%) were extracted and nine teeth (23.1%) functioned without problems after receiving root canal treatment. Conclusions: Within the limitation of and short-term evaluation, invasion of implant to adjacent tooth would have a minor influence on the prognosis of a tooth and outcomes of an implant when an appropriate root canal treatment was performed in cases of pulp necrosis through regular checkups. (JOURNAL OF DENTAL IMPLANT RESEARCH 2015;34(1):12-21)