Patients who stop smoking often complain of aphthous (mouth) ulcers. This symptom is sometimes attributed to the use of smoking cessation medications, but little is known about it. We investigated the incidence, severity, and time course of mouth ulcers in abstaining smokers and the effect of different smoking cessation medications on the symptom. The sample consisted of 1234 smokers who sought treatment at a large smoking cessation clinic, maintained at least 1 week of continuous biochemically validated abstinence, and provided usable data. Participants assessed their mouth ulcers by rating a mouth ulcer item added to the Mood and Physical Symptoms Scale. Subjects made ratings weekly on three occasions while still smoking and over 4 weeks following smoking cessation. After stopping smoking, some 40% of patients developed mouth ulcers, mostly in the first 2 weeks. The problem was generally mild, but 8% reported severe ulceration. The ulcers resolved within 4 weeks in 60% of patients affected. The ulcer ratings in patients using oral nicotine replacement products were higher than in those using patch, nasal spray or bupropion in the first week of abstinence but not afterward. Mouth ulcers were more prevalent in more dependent smokers, and the occurrence of ulcers correlated with other tobacco withdrawal symptoms. Our results confirm that mouth ulcers are a common result of stopping smoking, affecting two in five quitters. Patients should be reassured that the lesions are a result of stopping smoking and not a side-effect of smoking cessation medication.