Background. Argon-plasma coagulation is a method for tissue coagulation that uses high-frequency electric energy and ionized argon gas. It is used in endoscopic haemostasis and in coagulation of smaller, superficial lesion on gastrointestinal mucosa such as flat adenomas, but also in reduction of tumor tissue. Case report. We presented a patient with flat adenoma duodeni. Adenoma had been treated with argon plasma coagulation, in one act, and with a complete restitution of mucosa in further follows up. Conclusion. Argon plasma can be efficiently used in coagulation of superficial lesion of gastrointestinal mucosa, that belongs to the type of flat adenomas, as well as other superficial lesions of mucosa that require endoscopic mucosectomy from smaller, bordered spaces.