Primary gastric diffuse large B-cell lymphomas are generally well controlled by non-surgical treatment with combination chemotherapy followed by radiotherapy. We have previously reported that over 90% of patients achieved complete response (CR) with this therapeutic strategy: three cycles of cyclophosphamide, adriamycin, vincristine and prednisone followed by radiotherapy (40.5 Gy). Although the CR rate was very high, some patients still showed resistance to this combination therapy. In order to clarify the factors related to therapy resistance, we examined the relationship between Epstein-Barr virus (EBV), which was examined using an in situ hybridization technique, and the patients' clinical courses. Out of the 50 patients, four were EBV positive; over half of lymphoma cells were positive for EBV by in situ hybridization. Of the three EBV-positive patients, two showed progressive disease and one achieved partial response (PR). Two of the patients died of disease progression. The other patient achieved CR, but the lymphoma recurred with distant metastasis in the cerebellum 3 months after remission. In the present study, eight patients did not achieve CR or they relapsed, four patients showed progressive disease, one patient achieved PR, and three patients achieved CR with recurrence. Therefore, half of these unfavorable patients were EBV positive. This finding strongly indicated that EBV-associated gastric diffuse large B-cell lymphomas frequently show resistance to standard chemoradiotherapy, although some other adverse factors remain unclear.