Epstein-Barr virus (EBV) is associated with Burkitt lymphoma, nasopharyngeal carcinoma, and lymphoprolifcrative disease in patients with congenital or acquired immunodeficiencies. B lymphocytes in tissues from these patients often contain EBV genomes. Expression of the EBV genes EBNA-1, EBNA-2, and LMP and the B cell genes CD23, ICAM-1, and LFA-3 has been demonstrated in lymphoblastoid cells in vitro and in B lymphocytes in tissues from patients with lymphoproliferative disease. In contrast, B cells in tissue from patients with Burkitt lymphoma usually show expression of EBNA-1, but not the other viral or B cell genes. Expression of EBNA-1 with variable expression of LMP has been reported in tissue from patients with nasopharyngeal carcinoma. The different patterns of viral and cellular gene expression in these diseases have important implications for their treatment.