Pityriasis rosea eruption in secondary syphilis: an isomorphic phenomenon?

A 41-year-old man was referred because of a slightly pruritic eruption, and a classic case of pityriasis rosea was diagnosed on the first examination. Results of routine tests for syphilis were strongly positive, and on repeated examination the eruption had grown further. Skin biopsy specimens showed plasma cells and Treponema pallidum. All symptoms disappeared following antisyphilitic treatment. The possibility of an isomorphic response is discussed.