Some multi-walled carbon nanotubes (MWCNTs) induce mesothelioma in rodents, straight MWCNTs showing a more pronounced effect than tangled MWCNTs. As primary and secondary genotoxicity may play a role in MWCNT carcinogenesis, we used a battery of assays for DNA damage and micronuclei to compare the genotoxicity of straight (MWCNT-S) and tangled MWCNTs (MWCNT-T) in vitro (primary genotoxicity) and in vivo (primary or secondary genotoxicity). C57Bl/6 mice showed a dose-dependent increase in DNA strand breaks, as measured by the comet assay, in lung cells 24 h after a single pharyngeal aspiration of MWCNT-S (1-200 µg/mouse). An increase was also observed for DNA strand breaks in lung and bronchoalveolar lavage (BAL) cells and for micronucleated alveolar type II cells in mice exposed to aerosolized MWCNT-S (8.2-10.8 mg/m(3)) for 4 days, 4 h/day. No systemic genotoxic effects, assessed by the γ-H2AX assay in blood mononuclear leukocytes or by micronucleated polychromatic erythrocytes (MNPCEs) in bone marrow or blood, were observed for MWCNT-S by either exposure technique. MWCNT-T showed a dose-related decrease in DNA damage in BAL and lung cells of mice after a single pharyngeal aspiration (1-200 µg/mouse) and in MNPCEs after inhalation exposure (17.5 mg/m(3)). In vitro in human bronchial epithelial BEAS-2B cells, MWCNT-S induced DNA strand breaks at low doses (5 and 10 µg/cm(2)), while MWCNT-T increased strand breakage only at 200 µg/cm(2). Neither of the MWCNTs was able to induce micronuclei in vitro. Our findings suggest that both primary and secondary mechanisms may be involved in the genotoxicity of straight MWCNTs.