Resection specimens from 145 patients with primary B-cell gastric lymphoma at stage IE (n = 88) and at stage IIE (n = 57) were investigated. Histologically, low-grade malignant B-cell lymphomas arising from the mucosa-associated lymphoid tissue, including immunocytoma (n = 71), could be distinguished from high-grade malignant B-cell lymphomas with (n = 25) and without (n = 49) evidence of a low-grade component. The very rare low-grade B-cell lymphomas of centroblastic-centrocytic, centrocytic, and plasmacytic type were not considered. All patients had undergone primary gastric resection, and 65 received additional chemotherapy (n = 33), radiotherapy (n = 22), or both (n = 10). Actuarial overall survival rates calculated by the Kaplan-Meier life-table method were 76% after 5 years and 58% after 10 years. According to the Mantel test and a multivariate analysis using the Cox regression method, patients at stage IE had a significantly better survival probability than those at stage IIE (P less than 0.0001); 5-year survival rates were 87% and 61%, respectively. The survival probability for low-grade malignant lymphomas was significantly better than for tumors with secondary high-grade transformation (P less than 0.05) or for primary high-grade lymphomas (P less than 0.0001), whereas the two high-grade groups were not significantly different. Five-year survival rates were 91% for low-grade, 73% for secondary high-grade, and 56% for primary high-grade malignant lymphomas. Retrospectively, no significantly different survival rates were found between patients who had undergone gastric resection alone and patients who had received additional treatment. However, survival analyses showed that classification and grading according to the histopathological concept of mucosa-associated lymphoid tissue-derived gastric lymphomas into low-grade B-cell lymphomas of mucosa-associated lymphoid tissue type and high-grade B-cell lymphomas with or without evidence of a low-grade component has great prognostic relevance.