The biology of Hodgkin's disease is one of the most intriguing subjects in lymphoma research. The presence of only a small proportion of neoplastic cells and a vast majority of reactive cells reflects the presence of complex interactions between these cell types. In this paper we discuss findings indicating that Reed-Sternberg cells may be virally transformed cells that have evaded a cytotoxic immune response and induce an ineffective delayed type hypersensitivity reaction.