AIMS--To determine the presence of Epstein-Barr virus (EBV) genome in six patients (three with gastric and three with breast carcinoma) with severe small lymphoid cell infiltration. METHODS--The polymerase chain reaction and in situ hybridisation were used to detect EBV genome. The number and distribution of T and B lymphocytes were evaluated by immunohistochemistry. RESULTS--Histologically all of the patients had poorly differentiated tumours. Immunohistochemistry showed that T cells predominated in three cases, B cell in two, and almost equal numbers in one case. PCR showed that the EBV genome was present in two cases each of gastric and breast carcinoma. In situ hybridisation for EBV genome provided positive signals only in the small lymphoid cells in one gastric and two breast carcinomas giving a positive reaction for EBV genome by PCR. The gastric and breast cancer cells did not give positive signals. CONCLUSIONS--Severe lymphoid cell infiltration in gastric and breast carcinoma does not necessarily indicate that these tumours are associated with EBV. Larger numbers of cases will need to be studied to confirm this.