A number of reports have indicated that a hemophilia-like disease associated with pregnancy and the postpartum period is caused by a circulating anticoagulant directed against Factor VIII (antihemophilic globulin). This communication reports the clinical and laboratory findings of a primigravida who developed a hemorrhagic diathesis in association with such an anticoagulant. This anticoagulant was unique in that it was found in the patient's plasma prior to delivery. The inhibitory activity to Factor VIII was initially found in the IgM fraction, but analysis of subsequent plasma samples revealed a shift to the IgG fraction. The patient was managed successfully throughout delivery and the puerperium with infusions of Factor VIII concentrate sufficient to overcome the effect of the inhibitor.