Asbestos represents the main risk factor for malignant pleural mesothelioma (PM) even if only a minority of exposed people develop this tumor, suggesting a significant role for genetic susceptibility. This study aims to evaluate micronuclei (MN) frequency as a biomarker of genome instability in peripheral blood lymphocytes of PM and lung cancer (LC) patients when compared with healthy controls (HCs) and patients with nonneoplastic respiratory diseases (RDs). Lymphocyte cytokinesis-block MN assay was carried out on 317 subjects. Mutagen sensitivity, measured by quantifying MN frequency after an in vitro challenge with ionizing radiation, was evaluated in 252 subjects. A significant increase in MN frequency was observed in cancer patients compared to HCs, with a mean ratio (MR) of 1.35 and 1.36 at baseline and 1.43 and 1.38 after irradiation in PM and LC patients, respectively. A positive (synergistic) interaction between asbestos exposure and disease status was observed for MN frequency after irradiation in PM patients with possible exposure to asbestos (MR = 1.62). The evidence of increased genetic damage in cancer cases confirms lymphocyte cytokinesis-block MN assay as a sensitive predictor of the role of genetic instability in carcinogenic processes.