On the basis of cytologic studies of 144 patients with non-Hodgkin's malignant lymphomas (NHML), a cytologic classification was established that was composed of three groups (1, 2 and 3) containing nine classes (1A, 1B, 1C; 2A, 2B, 2C; 3A, 3B, 3C). Statistical analyses were carried out using 52 well-defined elementary cytologic characteristics. All the results given by the data-gathering procedures (hierarchical and dynamic clustering methods), by the descriptive analyses (multifactorial and canonical analysis) and by the stepwise discriminant analysis were in agreement with the proposed cytologic classification system. The most discriminating cytologic characteristics of the classes were the cell size, the nuclear-cytoplasmic ratio, the degree of cytoplasmic basophilia, the homogeneity of the cytoplasm and the number of mitoses. The use of these properties renders this classification system reproducible and applicable to clinical practice. A comparison was made between the cytologic classes and the NHML patients as grouped according to their clinical courses. Of the patients in classes 2A, 2B and and 2C, 94% showed acute leukemic characteristics. Of those contained in classes 3A, 3B and 3C, 78% showed poor prognosis or metastatic patterns. Of those contained in classes 1A, 1B and 1C, 76% showed a good prognosis pattern.