Human herpesvirus (HHV)-8 has been demonstrated to be involved in the pathogenesis of Kaposi's sarcoma, body cavity lymphomas, multicentric Castleman disease, and, more recently, acute bone marrow failure. In order to study the correlation between HHV-8 and immunosuppression, we performed a serological study in a group of 28 pediatric heart and heart-lung transplant recipients; of mean age 11.5 +/- 2.5 years. We analyzed blood samples prior to, at 3 to 6 months, and at least 12 months after transplantation. All patients were negative pretransplantation. We observed 10 seroconversions: one patient at 6 months; two patients at 12 months; and seven within the third year after transplantation. Our preliminary data demonstrated that HHV-8 seroconversion is frequent among pediatric transplant recipients. It was probably related to the length of immunosuppression rather than the organ transplantation; Serological assays of all donor specimens of seroconverted patients were negative.