This is the first case report of gastric mucosal-associated lymphoid tissue lymphoma with adult T cell leukemia/lymphoma (ATLL) cell infiltration. A 43-yr-old Japanese woman who was seropositive for antihuman T cell leukemia virus-I antibody complained of epigastric pain in April, 1996. Endoscopy showed gastric ulcers in the antrum. Biopsy specimens showed Helicobacter pylori infection. Her symptoms were relieved by treatment with ranitidine. In March, 1998, she complained of epigastric pain and abdominal fullness. Smears of peripheral blood revealed atypical lymphocytes with nuclear irregularity, consistent with ATLL cells. She was diagnosed to have ATLL. Endoscopy revealed multiple gastric ulcers in the antrum and the angle. Biopsy specimens demonstrated small centrocyte-like cells forming lymphoepithelial lesions, with infiltrations of large atypical lymphoid cells of ATLL. On immunohistochemical staining, the small centrocyte-like cells were positive for B cell markers (L26, CD20), whereas the large atypical lymphoid cells were positive for T cell marker (UCHL-1, CD45RO). Her findings were attributed to gastric mucosal-associated lymphoid tissue lymphoma with gastric involvement with ATLL.