The morphological and immunohistochemical characteristics of 37 atypical fibroxanthomas of the skin were examined. Twenty-four tumours were nodular ulcerative lesions on the head and face of patients with a median age of 75 years, whereas 13 tumours occurred on the trunk and limbs of patients with a median age of 48 years. Both pleomorphic polygonal and giant cells as well as the spindle cell component of the tumours stained for the histiocytic markers alpha 1-antichymotrypsin, alpha 1-antitrypsin, lysozyme and, less frequently, for ferritin. Leu M1 antigen and peanut agglutinin receptors were not demonstrable in tumour cells. This antigenic profile was contrasted with the findings in six cases of dermatofibroma which were largely not reactive with the antisera used. The immunohistochemical findings in atypical fibroxanthomas suggest that they represent a homogeneous group of tumours which are related to tissue histiocytes. These results are discussed in the context of the published findings in other so-called fibrohistiocytic tumours including dermatofibrosarcoma protuberans and malignant fibrous histiocytoma. The diagnoses in three cases coded as atypical fibroxanthomas were revised on the basis of their showing a different immunohistochemical profile.