Paraffin-embedded surgical biopsies from 50 patients with newly diagnosed diffuse large cell lymphoma (DLCL) were examined for proliferative activity and DNA aneuploidy by flow cytometry. These results were correlated with the clinical characteristics of these patients and the course of their disease. High proliferative activity, defined as less than 80% of cells in G0 or G1, was found to be the single most important pretreatment adverse prognostic factor in these patients. This relationship remained significant after correcting for poor performance status and advanced Ann Arbor stage, the other factors found to be associated with a shortened survival. DLCLs with high proliferative activity were more probable to present with extranodal involvement than those with lower proliferative activity. The mitotic count as determined by light microscopy did not correlate with flow cytometry-defined proliferative activity and may be a less accurate method for assessing this important biological characteristic in DLCL. DNA aneuploidy was detected in 62% of cases but did not appear to have any prognostic significance. Biopsies from patients who presented with lymphomatous bone marrow involvement, however, invariably demonstrated an aneuploid stemline. These results suggest that differences in proliferative activity may be an important biological basis for the variable prognosis seen in DLCL.