Diffuse large cell and immunoblastic lymphomas are a heterogeneous group of tumors. They differ phenotypically and genotypically, and their clinical course varies in aggressiveness. This study evaluates the prognostic significance of proliferative activity and compares it with immunophenotype, genotype, and clinical data. Proliferative activity was measured by Ki-67 antibody. In 22 cases of diffuse large cell and immunoblastic lymphoma high proliferative activity (greater than 40%) was associated with longer disease-free survival (average 42 months). In contrast, the patients with lymphomas exhibiting low proliferative activity (less than 40%) died shortly after diagnosis (average 8 months). These differences are statistically significant and indicate a need for large-scale prospective studies.