The aim of this study was to investigate the effect of third molar extraction on the periodontal status of the adjacent second molar. A total of 61 second molars in 31 adult periodontitis patients were examined. Among them, 27 second molars without adjacent third molars were included in the test group, and 34 second molars with third molar were included in the control group. Clinical parameters including plaque index, gingival index, and pocket depth and radiographic bone loss were measured around the second molar both in test and control group. The result showed that: (1) the mean plaque index and gingival index of control group were higher than these of the test group but the difference was not statistically significant, (2) the mean pocket depth of the control group was higher than the test group significantly at distal and buccal surface, (3) radiographic hone loss was greater in control group than test group significantly, (4) in Pearson correlation analysis between the age of extraction and radiographic bone loss in the test group, a positive relationship was shown(p<0.01). Within limitation of this study, it may be concluded that third molar extraction in periodontitis patients showed an improvement in periodontal status in contrast the patients group having third molar, therefore earlier a removal of third molar may minimize radiographic hone loss of the adjacent second molar.