We evaluated the frequency of micronuclei (MN) in peripheral blood lymphocytes of patients with pleural malignant mesotelioma (MM), lung cancer, benign respiratory diseases, and healthy controls. A significant increased frequency of MN was observed in patients with MM in comparison with all the other groups (median, 11.4 binucleated MN/1000 binucleated cells versus 5.1, 6.1, and 6.2, respectively). No association was found between MN and asbestos exposure. Recently, genetic susceptibility associated with asbestos exposure has been recognized in the development of MM. The presence of high frequency of MN in peripheral blood lymphocytes of patients with MM could represent a useful index of individual susceptibility to this tumor.