JC virus, a human papovavirus, is the causative agent of the demyelinating brain disease progressive multifocal leucoencephalopathy (PML). PML is a rare but fatal disease which develops as a complication of severe immunosuppression. Latent JC virus is harbored by many asymptomatic carriers and is transiently reactivated from the latent state upon immunosuppression. JC virus has a very restricted host range, with human glial cells being the only tissue in which it can replicate at reasonable efficiency. Evidence that latent human cytomegalovirus is harbored in the kidney similar to latent JC virus led to the speculation that during episodes of impaired immunocompetence, cytomegalovirus might serve as helper virus for JC virus replication in otherwise nonpermissive cells. We show here that cytomegalovirus infection indeed leads to considerable JC virus DNA replication in cultured human fibroblasts that are nonpermissive for the replication of JC virus alone. Cytomegalovirus-mediated JC virus replication is dependent on the JC virus origin of replication and T antigen. Ganciclovir-induced inhibition of cytomegalovirus replication is associated with a concomitant inhibition of JC virus replication. These results suggest that reactivation of cytomegalovirus during episodes of immunosuppression might lead to activation of latent JC virus, which would enhance the probability of subsequent PML development. Ganciclovir-induced repression of both cytomegalovirus and JC virus replication may form the rational basis for the development of an approach toward treatment or prevention of PML.