More than two thirds of all non-Hodgkin's lymphomas (NHL) are of low-grade malignancy. This group comprehends five histological types: lymphocytic, immunocytic, plasmacytic, centrocytic, and centroblastic-centrocytic lymphomas. The lymphocytic lymphomas are second in frequency among the biopsies. The most common lymphocytic lymphoma is the chronic lymphocytic leukemia (CLL). It is characterized by the proliferation of small lymphocytes. The quantity and arrangement of the constantly present paraimmunoblasts and prolymphocytes allow to distinguish three histological subtypes of CLL. CLL, prolymphocytic leukemia, and hairy cell leukemia may be composed of B- or T-cells. Mycosis fungoides, Sézary's syndrome and T-zone lymphomas are T-cell lymphomas. All NHL of low-grade malignancy show a proliferation of small to medium sized lymphoid cells. Some large blast forms may be intermingled among these smaller cells. The inhomogeneity of low-grade malignant lymphomas with regard to the size of predominant cells and the admixture of blast forms determine the higher ("intermediate") degree of malignancy of some types in the group of low-grade malignant lymphomas. The overgrowth of the intermingled blast forms probably leads to the transformation into a lymphoma of high-grade malignancy. This event happens in variable frequency in the various types of low-grade malignant lymphomas. NHL of low-grade malignancy occur almost exclusively in adults, whereas the high-grade malignant lymphomas are found in all age-groups.