Kaposi's sarcoma of the gingiva and skin developed in an HIV-negative renal transplant patient while he was receiving cyclosporine therapy. The Kaposi's sarcoma developed shortly after the patient had an acute infection with cytomegalovirus (CMV). Electron microscopy of the tumor's established cell line showed two types of virus-like particles. CMV DNA was identifiable in the cell line whereas infectious CMV could be isolated only after repeated passages (only after 3 months of culture). The other virus could not be identified, but did not appear to be either HIV or papilloma virus. The patient's tumor regressed after the discontinuation of cyclosporine therapy and the recovery from the acute CMV infection.