- ecurrent aphthous stomatitis (RAS) is one of the most common lesions of the oral mucosa seen in primary care. Aphthous ulcers aff ect up to 25% of the general population, and 3-month recurrence rates are as high as 50%.1 Minor aphthous lesions are the most common pre- sentation. Th ey appear as small, round, clearly defi ned, painful ulcers that heal in 10 to 14 days without scar- ring. Th ey often recur. Major RAS lesions are larger (> 5 mm), can last for 6 weeks or longer, and frequently leave scars. Th e third type of RAS, herpetiform ulcers, present as many small clusters of pinpoint lesions that sometimes coalesce to form large irregular ulcers. Th ey last for 7 to 10 days. Factors that predispose patients to RAS include infections, trauma, dryness, irritants, toxic agents, genetic factors, hypersensitivity, and autoimmune conditions. Systemic diseases, such as HIV, Crohn disease, Behçet syndrome, Reiter syndrome, and gluten-sensitive enteropathy, can appear as aph- thous stomatitis. Recurrent aphthous stomatitis can also result from a nutritional defi ciency, partic- ularly lack of iron, vitamin B3 (as in pellagra), vita- min C (as in scurvy), folic acid, or vitamin B12.2,4,5 A change in local regulation of the cell-mediated immune system after activation and accumulation of cytotoxic T cells might contribute to the local- ized breakdown of mucosa. Treatment with topical steroids and local anesthetics, as well as systemic steroids, can help in severe acute conditions. We describe three young, otherwise healthy, patients who had RAS for several years. All three were found to suff er from a vitamin B12 defi ciency. Replacement therapy with vitamin B12 led to com- plete recovery.