Epstein-Barr virus (EBV) has been found in most cases of rare gastric lymphoepithelioma-like carcinomas and a small but significant proportion of common gastric adenocarcinomas. The presence of EBV in gastric cancer has been detected by polymerase chain reaction and in-situ hybridization, the latter technique demonstrating EBV in every malignant epithelial cell. The carcinoma cells express EBNA1 but not the other EBNAs or LMP1. A single fused terminal fragment of the EBV genome was detected in each of the EBNA1-expressing tumours suggesting that the virus-positive gastric carcinomas represent a clonal proliferation of EBV-infected cells. EBV antibody titres were elevated in patients with virus-positive carcinomas and also in those destined to develop EBV-positive carcinomas. EBV-specific cytotoxic T-lymphocyte activity was retained in patients with EBV-positive carcinomas suggesting that the tumours can evade immunosurveillance possibly by only expressing the EBNA1 protein which is not able to be processed and presented. These results implicate EBV as one of the factors contributing to the development of gastric cancer.