Pemphigoid gestationis is a rare autoimmune vesiculobullous skin disease closely related to the pemphigoid group of blistering disorders. It is unique in that it is most frequently associated with pregnancy. Diagnosis is made on the basis of the presence of a subepidermal vesicle on routine histologic examination and of linear deposition of the third component of complement along the basement membrane zone of perilesional skin. Abnormal expression of major histocompatibility complex class II molecules in the placenta may trigger the initiating immunologic event in this organ, which is followed by cross-reactivity with an antigen present in the skin and the characteristic cutaneous signs of the disease. The immunogenetics of this disorder are interesting. A role for the involvement of major histocompatibility complex class II antigens seems likely because of the association with human leukocyte antigens DR3 and DR4. Major histocompatibility complex class III associations have also been observed, as has a high frequency of anti–human leukocyte antigen antibodies. We review the current understanding of the molecular biologic and immunogenetic properties of this disease and discuss treatment and potential impact on the fetus. (Am J Obstet Gynecol 2000;183:483-91.)