The distinctive low-grade B-cell mucosa-associated lymphoid tissue lymphoma (MALToma) of the stomach has been well characterized in recent years, but its relationship with the more commonly occurring large B-cell gastric lymphoma has not been clarified. This study aimed to elucidate their relationship. Among 48 consecutive cases of primary malignant lymphoma found in gastrectomy specimens, there were 10 cases showing coexistence of these two elements, which were further studied in detail. The high-grade component predominated in six cases, the low-grade component predominated in two cases, and the two components were intermingled in two cases. In the low-grade component, the small neoplastic cells possessed irregular nuclei (centrocytelike), and glandular invasion was a prominent feature. In the high-grade component, the blasts occurred in clusters or sheets, and often possessed plasmacytoid cytoplasm; glandular invasion was a rare event. In both components, the neoplastic cells frequently showed formation of nodules suggestive of colonization of reactive lymphoid follicles. Immunohistochemical studies showed that the neoplastic cells in the low- and high-grade components expressed the same class of immunoglobulin light chain in eight of the nine cases studied; staining in one case was unsatisfactory. Their intimate relationship as well as identical light chain restriction suggests that the high-grade component arises through blastic transformation of the low-grade component.