The current study was initiated to compare the anti-lymphoma activity and side-effects of prednimustine/mitoxantrone (PmM) vs cyclophosphamide, vincristine, prednisone (COP) in patients with advanced low-grade non-Hodgkin's lymphomas in way of a prospective randomized multicenter trial. Two hundred and forty-six patients with stage III or IV centroblastic-centrocytic (CB-CC (Kiel-classification)) or follicle center lymphoma (FCL (REAL classification)) and centrocytic (CC) or mantle-cell-lymphoma (MCL) were randomized for therapy with either PmM or COP and are fully evaluable for response and toxicity. PmM consisted of prednimustine 100 mg/m2/day on days 1-5 and mitoxantrone 8 mg/m2 /day days 1 and 2, while COP comprised cyclophosphamide 400 mg/m2/day on days 1-5, vincristine 1.4 mg/m2/day on day 1 and prednisone 100 mg/m2/day on days 1-5. Both regimens were repeated for a total of six cycles followed by an additional two courses for consolidation in responding cases and a subsequent second randomization for interferon alpha maintenance vs observation only. Overall response rates were comparable with 83% complete and partial remissions after COP and 84% remissions after PmM. PmM revealed a significantly higher rate of complete remissions (36 vs 18%, P < 0.006), the majority being achieved after four courses. The more rapid and possibly also more effective reduction of the lymphoma cell mass by PmM resulted in a tendency to a longer event-free interval for patients achieving remissions after PmM as compared to COP with estimated median event-free intervals of 31 vs 14 months, respectively (P=0.04). Separate analysis of lymphoma subtypes showed a tendency to a lower rate of complete remission in CC or MCL as compared to CB-CC or FCL (16 vs 30%, P=0.12, NS) while overall response rates were in a similar range (81 vs 85%). In both subtypes, PmM induced a higher rate of complete remission while overall response rates were comparable after PmM or COP. Treatment associated side-effects comprised predominantly myelosuppression and granulocytopenia in particular which was more frequently observed after PmM than COP (43 vs 31 %, P < 0.0001). This difference was clinically irrelevant, however, since serious infectious complications were encountered in less than 3% of cycles after both regimens. COP therapy was associated with a significantly higher incidence and degree of hair loss and complete alopecia (31 vs 2%) as well as of peripheral neurotoxicity (23 vs 2%). These data show that both PmM and COP reveal a high anti-lymphoma activity in patients with advanced stage non-Hodgkin's lymphoma. PmM appears advantageous with a higher rate of complete remissions and a better tolerability with regard to secondary side-effects. A longer follow-up is needed to assess the long-term effects of initial treatment on disease-free and overall survival and the impact on additional maintenance therapy with interferon alpha.