Clinical and prognostic relevance of the Kiel classification of non-Hodgkin lymphomas (NHL) was investigated in 1127 patients entering a prospective multicenter observation study. Survival of the 782 (69.4 per cent) patients with low-grade malignant NHL (lymphocytic lymphomas, predominantly B-CLL, LP immunocytoma, centrocytic lymphoma, centroblastic-centrocytic lymphoma) exceeded that of the 341 patients (30.2 per cent) with high-grade malignant NHL (centroblastic, immunoblastic, lymphoblastic lymphomas). Prognosis was best in centroblastic-centrocytic lymphoma and in B-CLL and least favorable in immunoblastic and lymphoblastic lymphomas. Survival of LP immunocytoma and centrocytic lymphoma patients was intermediate after 2 to 2.5 years of follow-up. Corresponding to histopathology, pattern of survival curves of low-grade malignant NHL (slow decline, no plateauing) differed from that of high-grade malignant NHL (rapid decline, subsequent plateauing). Prognosis of B-CLL was superior to that of LP immunocytoma. Stages I and II were more frequent in centroblastic-centrocytic lymphoma (21 per cent) than in LP immunocytoma (2.5 per cent) and centrocytic lymphoma (11 per cent). Ability of radiotherapy to induce stable complete remissions in stage III of centroblastic-centrocytic lymphoma indicates prolonged restriction of lymphoma to the lymphatic system. In immunoblastic and centroblastic lymphomas, stages I and II were diagnosed in 34 and 38 per cent of cases, respectively, but only in stage I/IE of centroblastic lymphoma prolonged remissions were achieved by radiotherapy. In advanced high-grade malignant NHL marked improvement of prognosis was solely possible by induction of complete remissions whereas in corresponding low-grade malignant lymphomas also partial remissions were prognostically relevant.