Marked thrombocytopenia developed during pregnancy in both identical twins mothers who had systemic lupus erythematosus (SLE) and also type IIB von Willebrand's disease (vWD). The proband's platelet count decreased in the third trimester of pregnancy. Large-dose gamma-globulin and prednisolone treatments were performed because of the suspicion of immune thrombocytopenic reaction associated with SLE. These treatments were not effective. Her platelet count returned to the normal range immediately after delivery. Postpartum examinations revealed the decreased ristocetin cofactor activity and the deficiency of large von Willebrand factor (vWF) multimers in preserved plasma samples from the third trimester. These abnormal findings improved after delivery. Investigation of family members revealed that the proband had inherited type IIB vWD from her mother. The other twin, who was also under treatment for SLE, became pregnant about 1 year after delivery in the proband and followed almost the same course as that observed in the proband. As bleeding tendency was observed a few days before delivery, a factor VIII concentrate (Haemate P) was administered to compete with her variant vWF. This concentrate could prevent the further decrease in her platelet count, thereby correcting the hemorrhagic tendency. It seems evident that factor VII concentrate would be effective in treating thrombocytopenia associated with type IIB vWD.