Epstein-Barr virus-positive mucocutaneous ulcer (EBVMCU) is a newly recognized disease entity characterized by EBV-positive atypical B-cell proliferation. EBVMCU is a localized self-limited disease that affects mucosa and skin, especially the oral cavity. EBVMCU develops in immunosuppressive patients, such as those with methotrexate (MTX)-administrated rheumatoid arthritis (RA). Here we clinicopathologically analyzed 12 EBVMCU patients in a single institution. All cases were administrated MTX for RA, and five cases occurred in the oral cavity. All cases except one had demonstrated spontaneous regression after withdrawal of the immunosuppressive agent. We found 4 of 5 cases in the oral cavity had preceding traumatic events in the same site within a week before the onset of EBVMCU. Although there is no detailed and large study that has analyzed the trigger of EBVMCU, a traumatic event would indeed be a significant trigger for EBVMCU in the oral cavity. The cases were histologically classified; six cases were diffuse large B-cell lymphoma-type, five were polymorphous-type, and one was Hodgkin-like lesion type due to morphological appearance and immunophenotype. The PD-L1 expression was also examined by two antibodies for PD-L1 (E1J2J and SP142). Both antibodies revealed identical results for PD-L1 expression, and three cases were positive for PD-L1. The application of SP142 for evaluating the immune status of lymphomagenesis has also been proposed. Nine of 12 cases were negative for PD-L1, which implies that most EBVMCU cases may be caused by an immunodeficiency, rather than an immune-evasion, mechanism. However, as three cases were positive for PD-L1, immune escape may underly the pathogenesis in a subset of EBVMCU cases.