This study was aimed to evaluate the frequency of recurrent aphthous stomatitis (RAS) within the 6-week period after quitting smoking. The study group consisted of 90 subjects. Oral, medical findings and tobacco habits were recorded for all subjects. Nicotine replacement therapy (NRT) and behavioral treatment were applied to some of the subjects by a family physician. All subjects were evaluated for their RAS and periodontal measurements on baseline, 1, 3, 6 weeks by a periodontist. While the subjects were in this smoking cessation programme, 64 of the 90 smokers successfully quit smoking within the 6 weeks and 26 smokers dropped out during the third week of the study. Point prevalence of RAS among the subjects on the first day of the quitting period and at the end of the first, third and sixth week after smoking cessation was 3.3% (3/90), 18.9% (17/90), 21.1% (19/90) and 17.1 (11/64), respectively. In the following weeks, aphthous ulcer point prevalence was significantly higher than the quitting level (p<0.05). As the time after quitting increased, the incidence of aphthous ulcer decreased. Of 64 patients, 35 (54.6%) completed the 6 weeks using NRT and 29 (45.4%) of them did not use any medication. The aphthous ulcer frequency observed in the patients taking NRT [11.4% (4/35)] was lower when compared with the subjects taking no NRT [24.1% (7/29)] (p>0.05). The results of this study confirm that RAS is a complication of quitting smoking. Further studies are needed to identify the effects of NRT on RAS. © Springer-Verlag 2007.