The histogenesis of atypical fibroxanthoma (AFX) and its relationship to malignant fibrous histiocytoma (MFH) are a subject of controversy. Many investigators have proposed that AFX may represent a reactive process, while others contend that it is a true fibrohistiocytic neoplasm, closely related to MFH. In an attempt to determine whether biologic differences between AFX and MFH may be accounted for at the cellular DNA level, we performed ploidy analysis on 14 cases of AFX by flow cytometry and compared our results with previously reported DNA analyses of MFH. Thirteen of the 14 lesions demonstrated diploid distribution of nuclear DNA, and only 1 case had an aneuploid population. This contrasts with prior data on MFH, the vast majority of which are aneuploid. Our results suggest that, despite histologic similarities, AFX may be distinguished from MFH on the basis of DNA content. These findings may be significant in understanding the biologic behavior of AFX.