Human herpesvirus 8 (HHV-8) is the causal agent of all forms of Kaposi's sarcoma, including the iatrogenic form that presents in solid-organ transplant recipients. A longitudinal study of HHV-8 seropositivity was conducted among a cohort consisting of children and adult solid-organ transplant recipients. Antibodies to HHV-8 lytic proteins were detected by an indirect immunofluorescence assay in serum samples of 100 transplant recipients. HHV-8 seropositivity increased significantly, from 5.3% before transplantation to 15.8% after transplantation (P<.01). Seropositivity was not related to the age of the patient or the type of organ transplanted. HHV-8 seroconversion occurred in both children and adult recipients. None of the seroconversion events was related to the source of the donor organ. These findings suggest that HHV-8 infection is not uncommon among both adult and children transplant recipients and that viral infection may be acquired from an outside source other than the transplanted organ.