Mesotheliomas are pleural, pericardial, or peritoneal neoplasms frequently associated with asbestos exposure, and it is estimated that over the next twenty years up to 80,000 new cases are expected in the USA alone. We found simian virus 40-like DNA sequences in 29 of 48 mesotheliomas studied (60%) and demonstrated simian virus large-T antigen expression in 13 of 16 specimens. The matching lung samples did not contain simian virus 40-like sequences; however, they contained asbestos. These findings are to our knowledge the first demonstration of a physical link between DNA virus-like sequences and human mesothelioma. We suggest that a simian virus 40-like virus may act independently or as a co-carcinogen with asbestos. Moreover, the selective large T antigen expression by mesothelioma and not by the surrounding pulmonary parenchyma may have both diagnostic and therapeutic implications.