The relationship between malignant mesothelioma (MM) and asbestos is well established, but the determinants of host factor susceptibility of MM are not. This study probes susceptibility issues by examining gender-related differences in the distribution of 417 thoracic and 42 abdominal cases of MM from 1988-1989 California Cancer Registry databases. The age-adjusted incidence rate ratio (IRR) for male/female thoracic MM was 6.9 (95% confidence interval [CI]; 5.0-9.6) consistent with greater occupational exposure among men. However, the IRR for male/female abdominal MM was 1.5 (95% CI: 0.6-3.6). Also, average age of onset for thoracic MM was greater than for abdominal MM. Thus, some abdominal MMs may be due to nonoccupational asbestos exposure, occurring over a lifetime, interacting with host factor susceptibility. This study gives impetus to research regarding the importance of host factors and nonoccupational asbestos exposure in the etiology of malignant mesothelioma.