The Epstein-Barr virus (EBV) is associated with nasopharyngeal lymphoepithelioma and certain types of lymphoma. We studied the presence of EBV in Japanese patients with esophageal and gastric carcinomas. The polymerase chain reaction was used for the detection of EBV DNA, in situ hybridization was used for the detection of latent EBV infection, and immunohistochemical studies were used for the detection of EBV-encoded latent membrane protein in formalin-fixed and paraffin-embedded surgical specimens. Positive reactions for EBV were shown in two of 31 cases of gastric carcinoma and in one of 30 cases of esophageal carcinoma; these reactions were detected by all of the above methods only in the carcinoma tissue specimens. The morphologic features of these three cases included undifferentiated carcinoma with a lymphoid stroma; these features were very similar to those of lymphoepithelioma in the nasopharynx. A negative reaction for EBV was demonstrated in each of the other 29 cases of gastric and esophageal carcinoma. The results suggest that EBV is associated with gastric or esophageal undifferentiated carcinoma with a lymphoid stroma, just as it is associated with nasopharyngeal lymphoepithelioma.