The purpose of this study was to investigate the effects of smoking on adult periodontitis after non-surgical periodontal therapy. The study population consisted of 40 patients with moderate to advanced periodontitis. Smokers(n=20) were defined as individuals smoking at least twenty cigarettes per day at the time of the initial examination. The non-smoking group(n=20) second and the fourth weeks after periodontal non-surgical therapy. The results were as follows; 1. Clinical indices including plaque index, gingival index, and pocket depth were decreased in both smoking and non-smoking group at the first, the second, and the fourth weeks. Especially, clinical indices of non-smokers were more significantly decreased than those of smokers. 2. Non-motile rods were increaseed and motile rods were reduced at the fourth week. spirochetes were reduced significantly in the non-smoking group at the fourth week. These results suggest that smoking play a minor role in adult periodontitis after non-surgical periodontal therapy.