Atypical fibroxanthoma belongs to the family of spindle-cell and pleomorphic neoplasms of the skin. The lineage of differentiation of this tumor and the means whereby it can be diagnostically separated from other similar morphologic entities have been controversial. In order to address these issues, the authors studied 30 spindle-cell and/or pleomorphic cutaneous tumors, including atypical fibroxanthomas (AFXs), superficial malignant fibrous histiocytomas (MFHs), dermatofibrosarcoma protuberans (DFSPs), sarcomatoid squamous-cell carcinomas (SCCs), spindle-cell malignant melanomas (MMs), and leiomyosarcomas, (LMSs). These cases were analyzed using a panel of eight antibodies and the immunoperoxidase technique. AFXs were reactive for vimentin, alpha-1-antichymotrypsin (AACT), alpha-1-antitrypsin (AAT), and cathepsin-B (CB) but failed to display cytokeratin (CK), epithelial membrane antigen (EMA), S-100 protein, and desmin. MFHs and DFSPs exhibited immunophenotypic profiles that were nearly identical to that just described. In contrast, SCCs were typified by positivity for CK and EMA; MMs exhibited uniform reactivity for S-100 protein; and LMSs contained desmin in four of five cases. A surprising result was the expression of S-100 by LMSs. Also, all tumors displayed at least one of the three proteolytic enzymes assessed in this study (AAT, AACT, and CB), demonstrating the relative diagnostic nonspecificity of these determinants. It is concluded that AFXs are "fibrohistiocytic" neoplasms, with substantial morphologic and immunohistochemical similarity to MFHs. The immunohistochemical classification of spindle-cell and pleomorphic tumors of the skin necessitates the use of antibody panels to assess the presence of markers that are characteristic of each diagnostic group.