Mantle cell lymphoma (MCL) is a malignant lymphoma associated with a relatively aggressive clinical course and a median overall survival time of 3-4 years. Treatment usually consists of combination chemotherapy, often including topoisomerase (topo) inhibitors such as doxorubicin, etoposide and mitoxantrone. Topo IIa is an enzyme that is needed whenever uncoiling of DNA is necessary during the cell cycle. The enzyme is a marker of cell proliferation. We analyzed the expression of topo IIa in relation to Ki-67 and the clinical outcome in patients with MCL. Biopsy specimens from 95 untreated patients enrolled in two multicenter trials (1975)(1976)(1977)(1978)(1979)(1980)(1981)(1982)(1983)(1984)(1985) were investigated immunohistochemically with monoclonal antibodies against topo IIa (Ki-S4) and Ki-67 (Ki-S5). Patients with low (0-10%) topo IIa expression had a median overall survival time of 49.0 months, compared to 17.0 months for patients with high (more than 10%) topo IIa expression. The Kaplan-Meier analysis showed a significant difference in the overall survival time related to the percentage of topo IIa (Po0.001) and Ki-67 (Po0.001) positive tumor cells. Multivariate Cox regression analysis revealed the expression of topo IIa as the most important prognostic factor (Po0.001) in MCL superior to the international prognostic index (IPI), the Ki-67 index and other clinical characteristics.