A high percentage of children with Wilms' tumor are cured with multimodal treatment. A small percentage of these children will develop second tumors, perhaps related to a genetic predisposition to neoplasia or possibly secondary to the treatment utilized for Wilms' tumor. Malignant mesothelioma has been associated with contact with asbestos but has also been reported after radiation exposure. Two patients are reported who developed malignant mesothelioma of the pleura after treatment for Wilms' tumor in childhood. Both received orthovoltage radiation; one patient also received triethylenemelamine (TEM), an alkylating agent closely related to nitrogen mustard, for 5 years. Factors in the development of second tumors are discussed.