405 cases with non-Hodgkin's lymphomas have been diagnosed according to the Kiel classification and analysed retrospectively. 314 patients with non-Hodgkin's lymphomas of low-grade-malignancy (chronic lymphocytic leukemia, lymphoplasmacytoid, centrocytic, centrocytic, centroblastic-centrocytic lymphoma) manifested significantly higher median survival times than the 91 patients with non-Hodgkin's lymphomas of high-grade malignancy (lymphoblastic and immunoblastic lymphoma). Within the group of patients with low-grade malignant lymphomas distinct prognostic differences were found whereas survival times in patients with lymphoblastic or immunoblastic lymphomas were rather similar. The lymphoblastic lymphoma showed a bimodal curve of age distribution whilst all other lymphomas had a maximum of incidence in the seventh decade of life. Increased frequency of B-symptoms did not necessarily represent an unfavorable prognostic factor for the lymphoma entity concerned. Except for chronic lymphocytic leukemia the highest incidence of initial bone marrow involvement was seen in lymphoplasmacytoid, centrocytic and lymphoblastic lymphomas. Centrocytes have been observed in peripheral blood of patients with centrocytic and centroblastic-centrocytic lymphomas, even though lymphocytosis did not exist. Monoclonal hypergammaglobulinemia was found in only 43% of the sera from patients with lymphoplasmocytoid lymphoma. In this disease, it was possible to differentiate between a lymphonodal, a splenomegalic and an extranodal manifestation.