Lichen planus (LP) is a chronic inflammatory disease of the skin, mucous membranes and appendages of the skin; the population frequency is estimated at 0.4-1.9%. Rashes with typical LP in the mouth and on the lips red border are asymptomatic and are represented by white laced patches or lines resembling a Wickham mesh. Exudative-hyperemic, erosive-ulcerative, bullous, infiltrative and atrophic forms represent atypical LP of the oral cavity and lips red border which frequency is comparable to the frequency of typical LP of the oral cavity; some of them can transform into squamous cell carcinoma. We present a description of the case of transformation into squamous cell carcinoma of erosive-ulcerative LP of the lower lip red border. Patient I., 75 years old, 5 years ago noticed a dense nodule on the lower lip red border that ulcerated six months later with the formation of a crust on the surface of the ulcer. Initially, the patient was observed by a dentist with cheilitis and leukoplakia and he used Solcoseryl ointment without effect. Later the dermatologist diagnosed the erosive and ulcerative form of LP of the lower lip red border. Repeated courses of treatment with oral corticosteroids lasting from one to several weeks resulted in only partial and temporary improvement. Gradually, a ridge-like zone formed around the ulcer; bleeding appeared during trauma and removal of the crust covering the ulcer that made it possible to suspect the transformation of LP into cancer. Clinical and histological signs of transformation of the erosive and ulcerative form of LP of the lips red border into cancer are discussed, as well as the compliance issues with oncological vigilance by dermatovenerologists and dentists in the management of patients with erosive and ulcerative form of LP of the lips red border.