A 79-year-old women with upper abdominal pain, vomiting and weight loss was found at endoscopy to have a large tumour mass in the gastric body. Histology of forceps biopsies revealed an adenocarcinoma of intestinal type. Gastrectomy was performed, but extensive lymph node metastasis precluded a curative surgical approach. Histopathological study of the specimen, however, revealed two distict malignancies, which arose in the setting of Helicobacter pylori-associated chronic gastritis with partial mucosal atrophy. One tumour was a gastric carcinoma, while the other was a primary B-cell lymphoma of the stomach (CD20-positive). The lymphoma comprised both a low-grade component (mucosa-associated lymphoid tissue- or MALT-type lymphoma), and a high-grade component (large cell lymphoma with CD30-positive giant cells). Infection with H. pylori was confirmed by the serological presence of IgG antibodies to H. pylori-antigens, including antibodies against the 128 kDa protein of the cytotoxin-associated gene (cagA gene) of H. pylori.