The purpose of the present study was to investigate the HH-8 seroprevalence among patients with Kaposi's sarcoma (KS), melanoma and gastric carcinoma (GC) as well as among renal recipients and blood donors. The obtained data revealed a high percentage of seropositive KS patients, which ranged from 83.6% in the classical disease type to 68.8% and to 71.4% in the immunosuppressive and AIDS-associated disease types, respectively. On the whole, the positive humoral response to HHV-8 reliably correlated with the positive findings if the viral genetic information in tumor tissue samplings. An unexpectedly high percentage of seropositive persons was found among the GC patients (41.8%) and among the renal recipients (26%), which is apparently predetermined by the immunosuppressive condition of such patients. Seroprevalence was found only in 4% of blood donors. Thus, the obtained data make it possible to conclude that KS cases, as diagnosed in Russia, are tensely associated with HHV-8 in spite of a low virus spread among the healthy population. Patients with pathology concomitant with a pronounced immunosuppression are characterized by a high prevalence of HHV-8 and belong to the category of persons with a KS risk.