Fourteen consecutive cases of atypical fibroxanthoma (AFX) seen during a 4-year period were studied histologically; of these, 12 were further examined for the presence of immunocytochemically detectable cytokeratin (CK), vimentin (VIM), S-100 protein, melanocyte-associated antigen (MAA), muscle-specific actin (MSA), alpha-1-antitrypsin (A1AT), alpha-1-antichymotrypsin (A1ACT), and ferritin (FER). In four cases, electron microscopy was also performed. Tumor cells were nonreactive with antibodies directed against CK and MAA, strongly reactive with anti-VIM, and variably reactive with A1AT, A1ACT, MSA, and FER. Our findings are consistent with the current notion that these tumors are "fibrohistiocytic". However, in 11 of 12 cases studied, a subpopulation of cells with features of Langerhans' histiocytes (LH) was also identified. These were dendritic cells within the substance of the tumor that were strongly reactive with S-100 antibody and uniformally nonreactive with MAA antibody; ultrastructurally, they were seen to contain typical Birbeck granules. LH characteristically comprised no more than 5% of the overall cell population of the tumor; however, in restricted portions of some lesions, they sometimes accounted for up to 80% of tumor cells. The occurrence of LH in AFX, although previously reported, has not generally been emphasized. Awareness of their presence as an expected and sometimes extensive component of AFX can be important when interpreting differential immunocytochemical panels applied to malignant spindle cell tumors of skin.