We report the simultaneous occurrence of Kaposi's sarcoma (KS) and primary cutaneous B-cell lymphoma (CBCL) of the leg in a 79-year-old woman, seronegative for HIV-1, HTLV-1 and HTLV-2. The CBCL underwent complete clinical remission after local radiotherapy, whilst the KS became disseminated within a year following diagnosis. However, 2 years after the diagnosis of KS, the patient died with neurological symptoms. These were presumed to be due to involvement of the central nervous system by lymphoma, although in the absence of an autopsy, this could not be proven. Skin biopsies from the original KS and CBCL lesions, as well as short-term culture of spindle cells from the KS lesion and peripheral blood mononuclear cells (PBMC), were studied by semiquantitative polymerase chain reaction (PCR) using primers specific for DNA sequences of a novel gamma-herpesvirus-8 (HHV-8). PCR studies were strongly positive for the virus on KS cells and PBMC; conversely, a low viral load was found on CBCL cells. A high titre of serum IgG antibodies reacting with the nuclei of the HHV-8 positive cell line BCP-1 was found. These data suggest that reactivation of latent infection with HHV-8 had occurred in this patient, and that HHV-8 is directly involved in KS, but not in CBCL of the leg, an aggressive variant of CBCL.