Adamantinoma of long bones is a rare malignant tumor composed of cells with epithelial characteristics in various differentiation patterns surrounded by fibrous cells. Evidence as to whether this neoplasm should be designated as an epithelial bone tumor or a biphasic sarcoma with both epithelial and mesenchymal features is lacking. In this study the nature of the mesenchymal and epithelial components of adamantinoma was investigated by DNA flow cytometry, DNA image cytometry, p53 immunohistochemistry, and polymerase chain reaction-based loss of heterozygosity detection at the p53 locus. Specimens from 6 of 15 patients (40%) analyzed by flow cytometry had an aneuploid DNA index. Image cytometry analysis of Feulgen-stained paraffin sections of 6 aneuploid and 2 diploid tumors revealed that aneuploid nuclei were detected in cells with an epithelial phenotype only, whereas all fibrous cells were diploid. Immunohistochemistry for p53 on specimens from 25 patients revealed moderate or strong immunoreactivity in 12 tumors (48%) restricted to the epithelial cells. Loss of heterozygosity at the p53 locus could be confirmed in the epithelial component of an immunohistochemically p53-positive tumor. Additionally, sections of 7 lung metastases were studied histologically. Only keratin-positive epithelial cells, predominantly in the spindle cell pattern, were present in these metastases, whereas the osteofibrous tissue present in the primary tumors was not detected. These results suggest that either adamantinoma consists of a malignant epithelial part with a reactive osteofibrous stroma or that the malignant epithelial cells develop next to a proliferating benign fibrous component. Additional analysis of common genetic abnormalities in the fibrous and epithelial cells of adamantinoma is therefore indicated.