A study of the occupational histories of 52 females with malignant mesothelioma and certain of their relatives, carried out to measure the risk of this disorder attributable to indirect asbestos exposure, showed that a significantly greater number of husbands and fathers of cases than of controls worked in asbestos-related industries, and the relative risk for this factor was 10. The frequency of parental cancer, especially gastrointestinal malignancy, was also significantly greater for cases than for their controls. This raises the possibility of a genetic predisposition to malignant mesothelioma.