The authors report a patient with severe secondary progressive MS who responded to mitoxantrone but developed a fatal acute myeloblastic leukemia 15 months after completion of mitoxantrone therapy. Therapy-related acute leukemia (TRAL) in relation with mitoxantrone is rare; this patient was the first case among a cohort of 802 French MS patients treated with mitoxantrone. Nevertheless, this case stresses the need to further evaluate the long-term risk of TRAL in patients with MS who receive mitoxantrone.