Although B-cell diffuse large-cell lymphoma (DLCL) can respond to chemotherapy and radiotherapy, a large number of patients are still resistant to treatment. Caspase-3 is an enzyme crucial to the apoptotic process and may be important in the clinical outcome of these patients. The pattern of caspase-3 expression was studied in 54 cases of DLCL using immunohistochemistry and quantitative reverse-transcription PCR. Tumor cells displayed both a diffuse cytosolic and a punctate cytosolic staining for caspase-3. Kaplan-Meier survival curves indicated that tumor cells with a diffuse cytosolic expression of caspase-3 correlated with a poor prognosis (P > 0.0004). In addition, a punctate cellular localization was associated with complete response to treatment (P = 0.011). Cases with a small percentage of lymphoma cells expressing caspase-3 also tended to show poor survival (P > 0.09). Levels of caspase-3 mRNA were not significant (P > 0.17), although a weak trend was observed similar to the immunohistochemical analysis. The pattern of expression of caspase-3 was also assessed with respect to terminal deoxynucleotidyl transferase-mediated nick end labeling (TUNEL) positivity in both reactive lymph nodes and B-cell DLCL cases. Our results suggest that TUNEL-positive cells are not caspase-3-positive and that there is no correlation between DLCL cases with a high degree of DNA fragmentation and caspase-3 immunostaining. Furthermore, a survival curve indicated that a high TUNEL positivity was associated with a poor survival probability (P < 0.02) and a poor response to treatment (P = 0.04). These results confirm the dynamic nature of caspase-3 expression in DLCL and suggests that the pattern of expression of the enzyme has prognostic significance.