The diagnostic findings of malignant histiocytosis (MH) were analysed in 12 consecutive patients in a single institution. Most patients presented with systemic symptoms and lymphadenopathy (92%), splenomegaly (100%) and hepatomegaly (67%). Neurologic symptoms were present in three patients, while involvement of other organs was present in five patients. The incidence of severe thrombocytopenia was 92% of anaemia 92% and of leucocytopenia 67%. Serum angiotensin converting enzyme, alpha 1-antitrypsin and lysozyme were independently increased in 6/9, 3/10 and 1/9 patients respectively. High serum levels of tumour necrosis factor (TNF) were present in 3/10 patients, while serum levels of interleukin-1 were normal in 10/10 patients. Histologic evidence of MH was obtained in all patients by repeated biopsies of involved tissues. Four patients died prior to treatment. Seven patients were treated with combination chemotherapy, consisting of CHOP (cyclophosphamide, doxorubicin, vincristine, prednisone) or MOPP (chloromethine, vincristine, procarbazine, prednisone), in some cases followed by non-cross-resistant second line chemotherapy, if no complete response was attained. The response rate of treated patients was 57%, and progression was observed in two patients. The median duration of response was 38 months. Three patients are alive without evidence of disease and off therapy (30+, 83+, 85+ months). Although MH is a potentially lethal disease, combination chemotherapy may offer a chance for cure in some patients.