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Primary extranodal malignant lymphomas are most often localized in the stomach. In contrast to gastric carcinomas, primary gastric non-Hodgkin's lymphomas show an increasing incidence. According to their grade of malignancy they are divided into low-grade and high-grade non-Hodgkin's lymphomas and according to their immunophenotype into B-cell and T-cell non-Hodgkin's lymphomas. In most cases they show a B-cell phenotype while high-grade tumors are more frequent than those of low-grade malignancy. However, primary gastric Hodgkin's disease is still a rarity. A new entity, the so-called low-grade B-cell non-Hodgkin's lymphoma of mucosa-associated lymphoid tissue (MALT) type, is characterized by a diffuse infiltrate of centrocyte-like cells intermingled with immunoblasts of the same clone, plasma cell differentiation of the tumor cells, lymphoepithelial lesions, and reactive intratumoral lymphoid follicles. It may secondarily transform into a high-grade B-cell lymphoma but remains limited to the stomach for a considerable period of time with a favourable prognosis. The most important prognostic factors of primary gastric lymphomas are stage at initial diagnosis, classification and grading according to the histopathological concept of the MALT, and depth of infiltration. Although a considerable number of early stage gastric lymphomas achieve complete remission after surgical therapy only, primary treatment of gastric lymphoma is still controverted, thus underlining the urgency of a multicenter prospective study. Chronic Helicobacter pylori infection may play a major role in the pathogenesis of low-grade B-cell lymphoma of MALT type. Complete remission of some cases of low-grade B-cell lymphoma of MALT type with concomitant Helicobacter pylori gastritis after Helicobacter pylori eradication may lead to a new pathogenetic, therapeutic, and prognostic concept.