Helicobacter pylori is an important pathogen involved in the development of gastrointestinal ulcers, but its involvement in oral ulcerous lesions is unclear. As culture is generally recognized as the gold standard for diagnosis of H. pylori infection, we employed this approach to assess the association of H. pylori with oral mucosal ulcerations. Samples were collected from patients with oral mucosal ulcerative disorders : 12 cases of recurrent aphthous stomatitis (RAS), 7 cases of herpes simplex virus (HSV) stomatitis, and 3 cases of erosive lichen planus (LP). Serum IgG antibodies against H. pylori were examined in all cases. All of the RAS and erosive LP cases were culturenegative for H. pylori, while two cases of HSV stomatitis were positive. The two culture-positive cases were also seropositve for the H. pylori antigen. It is suggested that H. pylori might not have a direct association with oral ulcerations. However, H. pylori in the oral cavity might exist in a non-culturable coccoid state without productive infection, and might form colonies only under special conditions such as HSV infection. (J. Oral Sci. 42, 225-2293, 2000)