A 29-year-old female in the 20th week of pregnancy was admitted because of a change in the ABO blood group and bleeding tendency. Acute myelogenous leukemia was diagnosed with a weak reaction of red blood cells with anti-A antibody and a decreased level of A-transferase activity. Though the patient tolerated intensive chemotherapy and achieved complete remission, thrombocytopenia persisted after consolidation chemotherapy. Since platelet-associated IgG was elevated, thrombocytopenia was considered to be immune-mediated. In the third trimester, premature separation of the normally implanted placenta developed and cesarean section was performed. The male baby was also thrombocytopenic, but successfully treated with gamma-globulin.