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The differential diagnosis of paratesticular malignant mesothelioma includes florid mesothelial hyperplasia, papillary serous tumors of the testis/paratestis, carcinomas of the rete testis and epididymis, and metastatic carcinomas. The distinction of malignant mesothelioma from florid mesothelial hyperplasia is discussed here. Papillary serous neoplasms arising in this location have broader papillae, with budding, nuclear stratification, and occasional ciliated cells. Psammoma bodies are more readily found and more numerous than in mesotheliomas. The tumor cells are usually positive for CEA and Ber-Ep4 - both of which are negative in mesotheliomas. Carcinomas of rete testis are generally centered around the testicular hilum and may show immunoreactivity for CEA. However, given the rarity of these tumors, clear-cut diagnostic criteria for their separation from mesotheliomas have not been fully established. Metastatic tumors generally involve the spermatic cord, epididymis and the testicular interstitium. Prominent vascular invasion is a frequent finding. The common primary sites are prostate and lung. Immunohistochemistry can usually resolve the diagnosis.