Thrombotic thrombocytopenic purpura (TTP) is in rare cases associated with antineutrophil cytoplasmic autoantibody (ANCA)-associated vasculitis, and often has a fatal outcome. We report the case of a 77-year-old woman with microscopic polyangitis (MPA) presenting with TTP. Rapidly progressive renal dysfunction and paralysis and sensory disturbance of the left lower limb were noted. Serum creatinine was 3.95 mg/dl, and the titer of myeloperoxidase-ANCA was 238 EU. She was diagnosed with MPA, and high-dose methylprednisolone was initiated, followed by 60 mg/day of prednisolone. Hemolytic anemia with red blood cell fragmentation, purpura, and thrombocytopenia developed during the course of active MPA. The activity of disintegrin and metalloproteinase with thrombospondin type 1 motifs 13 (ADAMTS13) was moderately decreased (27%). She was diagnosed with TTP, and plasma infusion was initiated, followed by plasma exchange (PE) with 40 units of fresh frozen plasma. Thrombocytopenia continued for more than a month (5-10x10(4)/microl). PE was repeatedly performed two or three times a week during the first 8 weeks from the beginning of PE in addition to prednisolone. Her clinical and laboratory findings gradually improved, and ADAMTS13 activity increased to 68%. The findings in this case suggested that ANCA-associated vasculitis may be involved in the development and the pathogenesis of TTP, and that repeated PE may need to be performed in addition to immunosuppressive therapy.