Pemphigoid gestationis (PG) is a rare auto-immune bullous disease of pregnancy that is characterized by linear deposition of C3 along the basal membrane zone of lesional, perilesional and uninvolved skin when examined by direct immunofluorescence. Circulating antibodies of IgG1 class are also present. The aberrant expression of MHC class II molecules in the placenta appears to be important in triggering the immune response in the lamina lucida of the basement membrane zone in the placenta which then cross-reacts with the skin. PG appears to be unique in that it produces both basement membrane zone antibodies and anti-HLA antibodies. Chronic placental insufficiency can occur so that there is a clear tendency for premature deliveries and small-for-date babies. Secondary auto-immune disorders are significantly increased in PG and appear to be related to the HLA antigens DR3 and DR4. Finally abnormalities of complement polymorphism have recently been described.