Electron microscopic analyses of tissue samples embedded in paraffin are routinely performed as a means of validating (establishing) the asbestos exposure of persons diagnosed with mesothelioma, a rare form of cancer often identified as being caused by exposure to asbestos. During analysis of tissue samples claimed to contain asbestos, we observed asbestos contamination in the paraffin of the tissue block. To investigate the extent to which such contamination is prevalent in tissue block samples, we obtained and analyzed samples of paraffin blocks from hospitals in six major cities and found them to contain measurable concentrations of asbestos. Whether this asbestos contamination originated in the virgin wax or was a result of processing has not been established. This result, which has not been previously reported, raises significant concerns about the validity of analyses for asbestos in tissue embedded in paraffin. In particular, diagnoses in which the presence of asbestos in tissue samples is taken as being indicative of past asbestos exposure, especially for those cases in which no known exposure has occurred, and studies purporting to show migration of asbestos to other organs in the body following inhalation or ingestion of asbestos require critical reevaluation. The need for reevaluation is particularly acute if appropriate control blanks were not evaluated as part of the studies.