Respiratory exposure to asbestos has been linked with mesothelioma in humans. However, its carcinogenic mechanism is still unclear. Here we studied the ability of chrysotile, crocidolite and amosite fibers to induce oxidative DNA damage and the modifying factors using four distinct approaches. Electron spin resonance analyses revealed that crocidolite and amosite containing high amounts of iron, but not chrysotile, catalyzed hydroxyl radical generation in the presence of H 2 O 2 , which was enhanced by an iron chelator, nitrilotriacetic acid, and suppressed by desferal. Natural iron chelators, such as citrate, adenosine 5′-triphosphate and guanosine 5′-triphosphate, did not inhibit this reaction. Second, we used time-lapse video microscopy to evaluate how cells cope with asbestos fibers. RAW264.7 cells, MeT-5 A and HeLa cells engulfed asbestos fibers, which reached not only cytoplasm but also the nucleus. Third, we utilized supercoiled plasmid DNA to evaluate the ability of each asbestos to induce DNA double strand breaks (DSB). Crocidolite and amosite, but not chrysotile, induced DNA DSB in the presence of iron chelators. We cloned the fragments to identify break sites. DSB occurred preferentially within repeat sequences and between two G:C sequences. Finally, i.p. administration of each asbestos to rats induced not only formation of nuclear 8-hydroxy-2′-deoxyguanosine in the mesothelia, spleen, liver and kidney but also significant iron deposits in the spleen. Together with the established carcinogenicity of i.p. chrysotile, our data suggest that asbestos-associated catalytic iron, whether constitutional or induced by other mechanisms, plays an important role in asbestos-induced carcinogenesis and that chemoprevention may be possible through targeting the catalytic iron. (Cancer Sci 2008; 99: 2142-2151) A sbestos fibers have been heavily used in industry from World War II to the present because of their durability, heat-resistance and low cost.(1,2) However, in 1987, the International Agency for Research on Cancer (IARC) designated asbestos fibers as a group I (definite) carcinogen for humans (http:// monographs.iarc.fr/ENG/Classification/crthgr01.php), and asbestos fibers were banned in many European and North American countries in the 1990s.(1,2) In June of 2005, asbestos-associated deaths suddenly attracted social attention in Japan because it was reported that 79 people who worked in factories using asbestos had been killed by an asbestos-associated rare cancer called diffuse malignant mesothelioma (DMM) over the past 26 years. Furthermore, people who lived near those factories also suffered from the same fatal disease.(4) The characteristics of DMM are as follows: (i) it is associated with repeated asbestos exposure; (5,6) (ii) once diagnosed, prognosis is poor; (7) and (iii) it takes 30-40 years after the start of asbestos exposure for DMM to occur.(5,6) It is expected that the incidence of DMM in Japan will peak in 2025 with 100 000 predicted deaths from this neoplasm. (8) There are several hypotheses a...