Herpes gestationis is a rare, pruritic, non-viral, polymorphic dermatosis of pregnancy and the puerperium. Lesions in this disorder are typified by pruritic vesiculobullae and urticarial plaques distributed over the abdomen, buttocks, and extremities. HG tends to recur in subsequent pregnancies and often flares at delivery. Although HG usually resolves within weeks to months of parturition, patients may experience a flare of disease after taking oral contraceptives or in response to other hormonal stimuli. Biopsy specimens from lesional sites demonstrate characteristic but not diagnostic features. The diagnosis of HG is based not only on its clinical and histopathologic features but also on the demonstration of linear deposits of C3 within the epidermal BMZ of the patient's normal-appearing, perilesional skin. These deposits signify the presence of a low-titer anti-epidermal BMZ IgG antibody that binds a precise antigen within the lamina lucida, avidly fixes complement, and presumably produces an inflammatory reaction responsible for tissue damage and blister formation. It is critical to study patient skin for evidence of C3 in situ because some patients with HG show no evidence of an antiepidermal BMZ antibody in routine indirect immunofluorescence microscopy and some remain negative in specialized complement-fixation studies. An additional laboratory finding in patients with HG is their increased frequency of the HLA-B8 and HLA-DR3 haplotypes as well as the HLA-DR3, -DR4 paired haplotype. Herpes gestationis has clinical, histologic, and immunopathologic features that resemble selected aspects of BP. Although many dissimilarities exist between these diseases, studies have shown that antiepidermal BMZ antibodies in patients with HG and BP recognize related or identical antigens.(ABSTRACT TRUNCATED AT 250 WORDS)