Recurrent aphthous stomatitis (RAS) is one of the most common oral mucosal diseases characterized by recurrent and painful ulcerations on the movable or nonkeratinized oral mucosae. Clinically, three types of RAS, namely minor, major, and herpetiform types, can be identified. RAS more commonly affects labial mucosa, buccal mucosa, and tongue. Previous studies indicate that RAS is a multifactorial T cell-mediated immune-dysregulated disease. Factors that modify the immunologic responses in RAS include genetic predisposition, viral and bacterial infections, food allergies, vitamin and microelement deficiencies, systemic diseases, hormonal imbalance, mechanical injuries, and stress. Our previous study found the presence of serum gastric parietal cell antibody, thyroglobulin antibody, and thyroid microsomal antibody in 13.0%, 19.4%, and 19.7% of 355 RAS patients, respectively. We also found anemia, serum iron, vitamin B12, and folic acid deficiencies, and hyperhomocysteinemia in 20.9%, 20.1%, 4.8%, 2.6%, and 7.7% of 273 RAS patients, respectively. Therefore, it is very important to examine the complete blood count, serum autoantibody, hematinic, and homocysteine levels in RAS patients before we start to offer treatments for RAS. Because RAS is an immunologically-mediated disease, topical and systemic corticosteroid therapies are the main treatments of choice for RAS.