A 52-year-old man developed malignant peritoneal mesothelioma 17 years after radiotherapy for seminoma of the testis. Although asbestos exposure is considered to be the major risk factor for the development of malignant mesothelioma, prior therapeutic radiation has also been postulated as a causative factor. The unexplained appearance of ascites or pleural effusion within a previously irradiated area should be considered suggestive of malignant mesothelioma in any long-term survivor of cancer. In addition, the patient suffered a deep vein thrombosis four years before the diagnosis of mesothelioma. Deep vein thrombosis is a common complication of malignant disease, and is often the first clue to occult malignancy.