Paraneoplastic pemphigus (PNP) is a form of pemphigus considered to bear characteristic immunohistopathological features (vacuolar-interface dermatitis, keratinocyte dyskeratosis and reactivity of patients’ sera with transitional epithelia). The present study was undertaken in order to investigate whether these criteria are specific enough so as to allow the diagnosis of PNP in the absence of clinical data. A retrospective study of 66 biopsies of pemphigus revealed that one third of them comprised histological signs of PNP; one of the corresponding sera reacted with mouse bladder epithelium. However, no evidence of a neoplastic disease was present in any of the patients, and Western blotting further excluded the diagnosis of PNP. These results suggest that some of the features considered characteristic of PNP are not strictly specific for this variety of pemphigus; hence this diagnosis cannot be reliably established by histology or immunofluorescence but requires biochemical studie