Several recent investigations using polymerase chain reaction (PCR) detection methods have identified SV40-like DNA sequences in human neoplasms, particularly choroid plexus tumors (1 ), ependymomas (1 ), mesotheliomas (2 ), and osteosarcomas (3 ,4 ). Difficulties have arisen because of the use of paraffin-fixed tissue, loss of possible viral episomes during DNA isolation, low copy numbers of viral DNA, PCR contamination, and confusion with BKV and JCV sequences. Nonetheless, the authentication of amplified products by DNA sequencing, the finding of a single 72-bp viral-enhancer repeat element, variability of large tumor antigen (T-ag) carboxy terminal DNA sequences in many specimens, and the isolation of SV40 virus from a choroid plexus tumor all support the validity of the basic observation that SV40 is present in humans, and may therefore contribute to oncogenesis in special situations.