We report here on a case of a 27-year-old woman in her first pregnancy. She was diagnosed with idiopathic thrombocytopenic purpura (ITP) at the age of 14 years. At 36 weeks of gestation, she was admitted to our hospital due to thrombocytopenia. We initially suspected ITP exacerbated by pregnancy. Laboratory results revealed mild anemia, thrombocytopenia (5.0×10(9)/l), and slightly elevated liver enzymes and lactate dehydrogenase. The next day, hemoglobin fell to 6.6 g/dl. Thrombotic thrombocytopenic purpura (TTP) was suspected on the basis of hemolytic anemia with schistocytes and a negative Coombs' test. Plasma exchange and methylprednisolone were initiated immediately. ADAMTS13 analysis showed a severe deficiency in ADAMTS13 activity but no inhibitors. At Day 6, the platelet count rose to 223×10(9)/l and she delivered a live baby by cesarean section. Currently, the patient receives fresh frozen plasma infusions every 2 weeks due to suspected Upshaw-Schulman syndrome.