INTRODUCTION: Mitoxantrone (Mx) is used as a second-line treatment in multiple sclerosis. Since 1998, eight cases of acute leukemia (AL) have been described. We report two new cases of myeloid AL that occurred during treatment with Mx. OBSERVATIONS: The first case concerned a women who was treated with Mx for 3 months. In spite of a very low total dose (58.32 mg), she developed promyelocytic AL. The second patient died of myeloid AL, 27 months after the last injection of Mx. DISCUSSION: All the reported cases of AL occurring after Mx respond to the criteria of leukemia induced by anti-topoisomerases II. Epidemiological data and those from animal experiments suggest that Mx has direct role in the occurrence of leukemia. CONCLUSION: It must be remembered that even if the risk of Mx-induced leukemia is low, blood cell counts must be closely monitored for at least five years after the last injection of this treatment.