A study of 138 patients with Non-Hodgkin's lymphomas was performed. The results were compared with retro- and prospective investigations of patients of the Kiel lymphoma study group. The aim of the study is to evaluate the value of the Kiel classification for the therapy and prognosis of malignant lymphomas. The malignant lymphomas of low- and high-grade malignancy differ significantly in their biological behaviour. Especially the clinical progress of the disease and the response to therapy are different. The lymphomas of low-grade malignancy may not need any treatment for long periods in some patients. In contrary, patients with lymphomas of high-grade malignancy need an active treatment immediately after the establishment of the diagnosis. In stages I and II radiotherapy is sufficient, in stages III and IV chemotherapeutic regimens are necessary. Each type of malignant lymphoma may be accompanied by leukemia. Most of the malignant lymphomas, particularly the immunocytomas, may produce monoclonal gammopathies, most of them being macroglobulinemias. Thus the macroglobulinemia Waldenström is just a clinical necessary to evaluate the significance of the subtypes of the Kiel classification for the planning of specific treatment.