A clinical example of the manifestation of vulgar pemphigus on the oral mucosa in the form of a single mildly painful erosion at the border of the soft and hard palate with extensive erosive lesions that existed in the patient for a long time is presented. The detection of a false positive reaction to syphilis led to an increase in the time of diagnosis. Only the appointment of a repeated full examination of the patient in accordance with clinical standards (cytological analysis, histological examination and indirect immunofluorescence method) allowed to establish the correct diagnosis, prescribe adequate therapy to the patient in a hospital setting and get a positive result.