Aphthous stomatitis is a common, recurrent, painful ulcerative condition of the oral mucosa. Cigarette smoking has been reported to protect against aphthous ulcers. To determine whether smokeless tobacco use also protects against aphthous ulcers, we examined the oral mucosa in 1456 professional baseball players, about half of whom were smokeless tobacco (ST) users. After controlling for the confounding effects of age, race, cigarette smoking, alcohol consumption, and dental hygiene practices, ST use was found to significantly reduce the risk of aphthous ulcers among these healthy young men (odds ratio = 0.4; p = 0.04). It has been suggested that cigarette smoking prevents aphthous ulcers by causing increased keratinization of the oral mucosa, and ST may protect by the same mechanism. Alternatively, a component of tobacco that is systemically absorbed might be responsible for protecting against aphthous ulcers. If the mechanism that protects ST users against aphthous ulcers is systemic, then nicotine is the likely protective factor.