In patients with autoimmune diseases malignancy is observed more frequently than in an age-matched control group. In strains of mice susceptible to autoimmune diseases, the incidence of lymphoreticular malignancies is increased. Although viruses have not been directly implicated in their pathogenesis or etiology, there are suggestions that slow viruses may be involved. Viruses have been found to be important in the development of lymphomas. Hence in genetically susceptible animals, viruses can initiate the autoimmune process. Prolonged stimulation of the autoimmune process or a prolonged state of immunodeficiency could trigger or activate genes or their products, which could result in the development of malignancy.