The clinical picture and course of lichen planus (LP) on the mucous membrane of the oral cavity have significant differences from its manifestations on the skin. In particular, greater resistance to treatment, which is due to the structural features of the mucous membrane, as well as the specificity of biological and physicochemical processes in the oral cavity. There are 6 clinical forms of LP of the red border of the lips and oral mucosa – typical, hyperkeratotic, exudative-hyperemic, erosive – ulcerative, bullous and atypical. The variety of clinical manifestations and low effectiveness of treatment present certain difficulties and interest not only for dentists, but also for dermatologists in terms of diagnostics and treatment of LP with localization on the oral mucosa. A clinical case of a rare triad of symptoms of erosive lichen planus (LP) of the oral cavity, hypertension and diabetes mellitus – Grinshpan-Potekaev syndrome is presented. In the described clinical case, a 65-year-old female patient took antihypertensive drugs for a long time for arterial hypertension. Type 2 diabetes mellitus was latent and was diagnosed after contacting our clinic. Complex therapy with Delagil, tetracycline, application of corticosteroid preparations and protopic cream (on the oral mucosa) along with effective antihypertensive and antidiabetic treatment led to a rapid regression of the manifestations of LP with complete epithelialization of erosions.