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Rupioid syphilid is a type of pustular syphilis which accounts for <2% of cases of secondary syphilis. In pustular syphilis, pustules undergo central necrosis giving rise to “limpet-like” crusts resembling an “oyster shell.” When the clinical findings are atypical, a diagnosis is made based on serologic testing. The prozone phenomenon occurs in <1% of patients with primary or secondary syphilis. This is a false negative reaction that occurs when very high antibodies in the serum prevent the formation of the antibody-antigen lattice needed to visualize a flocculation reaction. We report a case of rupioid syphilid in a pregnant woman accompanied by the prozone phenomenon. An accurate diagnosis was delayed because of atypical clinical findings and false-negative serologic results. (Korean J Dermatol 2011;49(6):557∼560)