The murine acquired immunodeficiency syndrome is induced by a defective retrovirus. To study the role of virus replication in this disease, helper-free stocks of defective Duplan virus were produced. These stocks were highly pathogenic in absence of detectable replicating murine leukemia viruses (MuLVs) other than xenotropic MuLV. They induced expansion of the infected cell population (over 1000-fold), and this cell expansion was oligoclonal in origin and, most likely, arose through cell division. These results suggest that this defective virus is oncogenic, inducing a primary neoplasia associated with an acquired immunodeficiency syndrome as a paraneoplastic syndrome. These data emphasize the need to determine whether virus replication is necessary for the progression of other immunodeficiency diseases, including acquired immunodeficiency syndrome, and whether these diseases also represent paraneoplastic syndromes.