We studied the diagnostic significance of immunohistolochemical staining and immunoglobuline gene rearrangement of jumbo-biopsy specimen from 14 patients with stomach lesions which were difficult to distingwish between reactive lymphoreticular hyperplasia (RLH) and mucosa-associated lymphoid tissue lymphoma (MALT lymphoma). We also investigated Helicobacter pylori (HP) infection in the patients and followed their clinical courses from a mean observation period of 3 years and 4 months following initial diagnossis. As a result, 7 of the 14 cases were diagnosed as having MALT lymphoma. All of them were resected, and then the diagnosis was confirmed. Metastasis was found in 2 cases. The other 7 cases were diagnosed as RLH. A favorable prognosis during follow up without any treatment suppoted the belief that they were nonmalignant lesions. HP infections were observed on 83% of the RLH cases and 57% of MALT lymphoma cases. In one of the case of RLH, the lesion disappeared after eradication of HP.