The association of the Epstein-Barr virus and two human malignancies, Burkitt's lumphoma and nasopharyngeal carcinoma, is reviewed. Seroepidemiologic, virologic, and immunologic evidence is summarized, and several hypotheses regarding a possible etiologic role for the Epstein-Barr virus in these tumors are presented. With our current state of knowledge we cannot conclude that the Ipstein-Barr virus is oncogenic in man, nor can we yet ascertain the biological and clinical significance of its association with Burkitt's lymphoma and nasopharyngeal carcinoma.