Twenty-five cases originally diagnosed as malignant histiocytosis/true histiocytic lymphoma were reviewed according to both pathological and clinical criteria. Microscopically, they were characterized by large, pleomorphic tumour cells showing variable degrees of atypia and phagocytic activity. The growth more often appeared as diffuse, being limited to the sinuses in only two cases. Cytochemistry on touch imprints showed tumour cells strongly positive for acid phosphatase and alpha-naphthyl-acetate esterase in all the samples tested. Immunohistochemistry on paraffin embedded sections using specific antisera showed tumour cell positivity for lysozyme in 12 of 25 cases, for alpha 1-antitrypsin in 24 of 25 cases and for alpha 1-antichymotrypsin in all 25 cases. Immunophenotyping on frozen-sections in three cases displayed a clear-cut reactivity of the neoplastic cells with the monoclonal antibody OKM1. Clinically, the disease more often presented with B-symptoms, lymphadenopathy and mediastinal involvement. In the majority of the patients (18/25) it had a fatal and rapid course, despite therapy (median survival: 9 months; mean survival: 12 months). The presence of B-symptoms and bulky disease appeared as the only factors influencing the prognosis, both suggesting a more aggressive course of the tumour.