Malignant adenolymphoma.

Adenolymphoma (Warthin's tumor) is a well studied benign tumor of the salivary gland. Malignant transformation of such a tumor is rare and not well documented in the literature. The light microscopic and ultrastructural features of an undifferentiated carcinoma arising in an adenolymphoma in the parotid gland of a middle aged male are described, and the relevant literature is reviewed. Similarities between the benign adenolymphoma and the undifferentiated malignant tumor, such as the presence of interstitial lymphoplasmacytic cell infiltrates, dark and light epithelial cells, similar cytoplasmic organelles, and nuclear morphology, suggest a malignant transformation of a previously existing benign adenolymphoma.