The non-Hodgkin's lymphomas of high-grade malignancy (NHL-hM) occur in patients of every age. There are two peaks of frequency: the first one between the 3rd and the 14th and the second one between the 50th and the 80th years of age. The NHL-hM may arise de novo or they develop more frequenctly in the course of a low-grade malignant lymphoma or in immune-deficiency states. They grow very fast and metastasise very early. However an early diagnosis allows to detect them in patients with the clinical stages I-II. By a radical operative removal of these tumours and by a thorough subsequent treatment a persistent remission or cure may be achieved. The most frequent types of NHL-hM are the lymphoblastic leukemias of childhood, the centroblastoma, and the immunoblastoma. In typical cases the histological diagnosis is easy. The immature, anaplastic types are difficult to diagnose and different methods (demonstration of surface and cytoplasmic immunoglobulins, cytochemistry, electron microscopy) most be applied. The most important differential diagnoses are the acute myeloblastic and myelomonocytic leukemias (naphthol-AS-D-chloroacetatesterase) immature epithelial tumours (lymphoepithelioma), and reactive processes (e.g. infectious monocleosis).