Mesothelioma developed in proximity to the field of therapeutic radiation administered 10-31 years previously in four patients. In three, mesothelioma arose within the site of prior therapeutic radiation for another cancer. Mesothelioma in the fourth patient developed adjacent to the site of cosmetic radiation to a thyroidectomy scar. None of these four patients recalled an asbestos exposure or had evidence of asbestosis on chest roentgenogram. Lung tissue in one patient was negative for ferruginous bodies, a finding considered to indicate no significant asbestos exposure. Five other patients with radiation-associated mesothelioma have been reported previously, suggesting that radiation is an uncommon cause of human mesothelioma. Problems in the diagnosis of radiation-associated mesotheliomas are considered.