Three malignant rhabdoid tumors of the kidney and two extrarenal rhabdoid tumors of soft tissues were studied by light and electron microscopy and by immunocytochemistry for the expression of keratin, vimentin, desmin, neurofilament triplet proteins, epithelial membrane antigen, myoglobin, and HNK-1 (Leu-7). Electron microscopy revealed the characteristic cytoplasmic whorled filamentous inclusions in all tumors. An epithelial phenotype (presence of cytoplasmic tonofilaments) was observed in two tumors (one renal and one extrarenal); and a focal primitive neural phenotype (cytoplasmic processes with neurosecretory granules), in a renal rhabdoid tumor. Strands of basal lamina were seen in two renal and one extrarenal rhabdoid tumors. Evaluation of basal lamina was more difficult in the third renal rhabdoid tumor, in which tissue preservation was not optimal. Primitive attachments were present in all rhabdoid tumors. Immunocytochemical staining supported a diverse phenotype, ranging from epithelial and/or mesenchymal to myogenous and/or neuroectodermal. Simultaneous expression of several of the studied antigenic determinants by the same tumor was noted. The findings suggest that both renal and extrarenal rhabdoid tumors express a diverse morphological and immunocytochemical phenotype.