BACKGROUND AND DESIGN: A previous study has suggested that there is a novel entity among the polymorphous eruptions of pregnancy (PEP) associated with circulating anti-basement membrane zone IgM autoantibodies. To determine if the presence of anti-basement membrane zone IgM autoantibodies is a feature of PEP, serum samples from 52 patients with a PEP, 69 healthy pregnant women, and 42 nonpregnant women were prospectively evaluated by indirect immunofluorescence using salt-split human skin as substrate. Serum samples were also tested by immunoblotting using keratinocyte extracts and anti-human IgM antibodies. The reactivity of some serum samples was examined using two recombinant bullous pemphigoid antigen proteins. RESULTS: The percentage of women with a PEP, healthy pregnant women, and nonpregnant women who had anti-basement membrane zone IgM antibodies by indirect immunofluorescence was similar: 12%, 10%, and 14% of cases, respectively. By immunoblotting, 14% of the serum samples from the patients with a PEP, 12% of the serum samples from the healthy pregnant women, but only 2% of the serum samples from the nonpregnant women contained IgM antibodies that reacted with epidermal proteins of 180 and/or 230 to 240 kd. The recombinant bullous pemphigoid antigen proteins were not recognized by any of the serum samples that showed a reactivity by immunoblotting using keratinocyte extracts. CONCLUSION: There is no evidence for the existence of a novel entity of pregnancy defined by circulating anti-basement membrane zone IgM autoantibodies. Immunoblotting detects IgM autoantibodies that react with epidermal proteins of 180 and/or 230 to 240 kd. These autoantibodies appear to be more frequent in pregnant than in nonpregnant women. Although the nature of the target antigen(s) remains to be established, pregnancy may be associated with low levels of IgM autoreactivity against epidermal proteins.