The ubiquitous, DNA herpesvirus, EBV, has B cell tropism and the geographically restricted RNA retrovirus, ATLV/HTLV-I has T cell tropism. Clinical descriptions by Burkitt and Takatsuki led to discovery of these viruses which infect silently early in life; however, ATLV is also transmitted to a spouse or by blood transfusion. In normal seropositive persons both viruses infect only 1 in about 10,000 B or T cells, respectively. EBV is associated with Burkitt's lymphoma, nasopharyngeal carcinoma, and infectious mononucleosis. ATLV is associated with adult T cell leukemia/lymphoma and smoldering T cell lymphoma. EBV infects polyclonally and is controlled by multiple cellular and humoral control mechanisms. Escape from immune surveillance as in immune deficient African children with malaria, males with x-linked lymphoproliferative syndrome, organ transplant recipients, and AIDS patients permits conversion from polyclonal to oligoclonal and finally, monoclonal malignancy. T cell immune defects permit proliferation of cells which undergo molecular and/or cytogenetic alterations. In contrast to EBV, which is integrated and nonintegrated in B cells, ATLV is monoclonally integrated. Viral transforming proteins and immune suppressive substances are produced. Immune deficiency in silent carriers of ATLV and in those with smoldering ATL suggest that immune surveillance deters emergence of ATL. Prevention of primary infection by vaccination against these lymphotropic viruses, and use of immunotherapy and antiviral drugs may potentially retard conversion of infected B or T cells to monoclonal malignancy.