Epstein-Barr virus has been demonstrated in angiocentric immunoproliferative lesions. However, these studies, which have used the polymerase chain reaction technique or Southern blot analysis, have not revealed the identity of the cell population harboring the virus. To address this issue, we analyzed 12 angiocentric immunoproliferative lesions (14 biopsy specimens) for Epstein-Barr virus RNA using a sensitive and specific in situ hybridization method. Epstein-Barr virus RNA was detected in five of 12 cases. Each positive lesion was a high-grade angiocentric immunoproliferative lesion (i.e., angiocentric lymphoma) in which Epstein-Barr virus was identified in numerous cells. Furthermore, the Epstein-Barr virus-positive cells were often larger than normal lymphocytes and thus were most likely the malignant cells. Double-labeling immunohistochemistry/in situ hybridization studies revealed that the majority of Epstein-Barr virus-positive cells were CD43-positive and CD20-negative, consistent with T-cell lineage. The remaining seven cases, all grade I or II angiocentric immunoproliferative lesions, were either completely negative (four cases) or had only rare to occasional Epstein-Barr virus-positive cells (three cases). The results obtained with the polymerase chain reaction and Southern blot hybridization studies generally confirmed those obtained with in situ hybridization. We conclude that Epstein-Barr virus is frequently associated with angiocentric immunoproliferative lesions, particularly within high-grade lesions, in which the virus is probably present within the neoplastic cells. These findings also suggest that Epstein-Barr virus may be involved in the transformation of low-grade angiocentric immunoproliferative lesions to histologically unequivocal angiocentric lymphomas.