The effect on periodontal tissues of lower third molar surgery, due to impaction or semi-impaction, has been investigated in a retrospective study comprising 215 cases. The post-operative examination took place 2 years after the surgical treatment and included both clinical and radiographic variables. Clinical registrations included the amount of plaque, and presence of gingivitis and periodontal pockets. The results showed a higher incidence of plaque, gingivitis and pockets on the distal surface of the second molar than on other surfaces of the first and second molars. The alveolar bone level distal to the second molar was registered by radiographic examination with a periodontal probe as indicator. 2 years post-operatively, 43.3% of the cases exhibited pocket depths exceeding 7 mm and 32.1% showed intrabony defects exceeding 4 mm. Some factors affecting the periodontal healing after lower third molar surgery are discussed.