Large-cell non-Hodgkin's lymphomas (T- and B-immunoblastic, centroblastic and true histiocytic lymphomas) have a heterogeneous clinical course. In the present study the clinical and morphological data of 20 cases of histiocytic sarcoma (true histiocytic lymphoma) are presented. Diagnosis was supported by immunohistochemistry, cytochemistry, rosette assays and/or electron microscopy. Although the follow-up was relatively short (up to 144 months, mean 26 months), the clinical data differed clearly from the series of large-cell non-Hodgkin lymphomas, recorded in the literature. Differences were found in age distribution with a peak in the third decade, in organ involvement showing a preference for skin, gastrointestinal tract and bone, and in response to therapy. In general, histiocytic sarcoma appears to have a more favourable response to therapy and clinical course than the other large-cell lymphomas (T- and B-immunoblastic and centroblastic lymphomas). Moreover, preliminary observations in the group of histiocytic sarcomas suggested that the presence of lysozyme and/or 5-nucleotidase and the absence of alpha 1-antitrypsin in the cytoplasm is associated with a better response to therapy and favourable clinical course.