We present our own case of the rare Hailey—Hailey disease (HHD) with three round lesions up to 0.5 cm in diameter on the mucous membrane of the hard palate, with negative Nikolsky sign, absence of acantholytic cells in the smears and of antibodies to desmosomes and the epidermal basal membrane during immunofluorescence studies. A histological examination of a few crusty erosions on the scalp and chest revealed supra-basal acantholysis with the formation of lacunae and blisters, which made it possible to diagnose HHD. The presented case reflects the difficulties of diagnosing HHD in the absence of genealogical data, the late (48 years) onset of the disease, atypical localization of rashes, and demonstrates the importance of an integrated approach in the diagnosis using histological and immunofluorescence studies. Extremely rare cases of lesions of the mucous membrane of the oral cavity, esophagus, vagina, and conjunctiva are initiated by trauma or infection. HHD is a relatively rare genodermatosis, which is inherited by a dominant trait with complete penetration; caused by mutations inactivating one allele of the ATP2C1 gene. Mechanical, temperature, chemical, allergic, bacterial, radiation, and other impacts provoke exacerbations (usually in summer). The treatment is difficult. One of three therapeutic approaches is recommended: 1) reduce friction and sweating; use emollients with antibacterial additives; soap substitutes; antiseptic bath additives; local corticosteroids of moderate to high activity in combination with topical or oral antibiotics and/or antifungal drugs to control secondary infection; pain relief (topically or systemically); 2) local corticosteroids of ultrahigh pharmacological activity in combination with topical or oral antibiotics and/or antifungal drugs to control secondary infection; 20—30 mg / day prednisone with a gradual decrease to control exacerbations; 3) antistaphylococcal antibiotics (efficacy not proven); locally — tacrolimus; or tacalcitol; or calcitriol; or 5-fluorouracil; systemic cyclosporin; or retinoids; or methotrexate.