Five cases of malignant rhabdoid tumors in adult patients are described. The tumors were characterized by sheets of poorly cohesive cells with distinctive hyaline cytoplasmic inclusions and eccentrically placed large vesicular nuclei with prominent nucleoli. A variety of lineage differentiation was shown by immunohistochemistry and electron microscopy, the lesions representing leiomyosarcoma, gastric carcinoma, epithelioid sarcoma, Merkel cell carcinoma and desmoplastic small round cell tumor. Previously reported cases of rhabdoid tumors in adults support the contention that such tumors, although of distinctive morphologic phenotype, are of heterogenous lineage differentiation, and do not represent a distinct clinicopathologic entity.