Objective: Analysis of the results of observation of clinical cases of diseases of the oral mucosa in order to differentiate existing foci for correct diagnosis and subsequent treatment. Materials and methods: Variants of pathological changes related to the group of white lesions: multiform erythema, flat lichen, candidiasis glossitis at the stage of formation of white foci. Results: In this paper we consider the differential diagnosis of multiform erythema, planar lichen, candidiasis glossitis in the presence of signs of white plaque. Clarification of the diagnosis is based on a thorough examination and additional (including laboratory) research methods. Conclusion: The analysis of clinical cases of diseases of the oral mucous membrane indicates the similarity of their elements of destruction and manifestations of other symptoms, which greatly complicates the diagnostic search. A thorough survey, inspection, consultations of general specialists and additional laboratory tests allow to successfully solve the tasks.