Recently, DNA sequences of what appear to be a unique human herpesvirus-like agent (HHV-8) have been detected in different types of Kaposi's sarcoma (KS) tumors (Chang, Y., E.C. Cesarman, M.S. Pessin, F. Lee, J.C. Culpepper, D.M. Knowles, and P.S. Moore. 1994. Science (Wash. DC). 266:1865-1869). To further elucidate the possibility that HHV-8 plays a role in the pathogenesis of KS, the expression of HHV-8 RNA was examined in fresh KS tissue specimens which were found to harbor HHV-8 DNA by polymerase chain reaction (PCR). The transcription of HHV-8 RNA was detected by RT-PCR in 26 of 29 specimens (89.7%) of the KS tumors including 2 of 3 CKS and 24 of 26 AIDS-KS. No positive signal was detected in eight biopsy specimens of normal skin from healthy donors. By Northern blot analysis, the expression of HHV-8 was detected in 2 of 10 KS tumors examined. Furthermore, the RNA transcripts were observed in endothelial cells lining the irregular vascular spaces and perivascular spindle-shaped cells histologically characteristic of KS in 2 out of 8 different KS specimens examined by in situ hybridization using an antisense probe specific of HHV-8. The detection of RNA expression of HHV-8 in KS tumors further supports the possible etiopathogenic role of this virus in the development of KS.