The effects of endoscopic microwave coagulation therapy on duodenal adenoma in a 58-year-old man are reported. The patient had a 35-mm sessile tubulo-villous adenoma in the duodenal bulb. Endoscopic polypectomy followed by microwave coagulation therapy for duodenal adenoma was performed to avoid a major surgical procedure which would disturb the physiological functions of the gastrointestinal tract. After partial polypectomy which included the core of the adenoma, the entire remaining adenoma was destroyed after three microwave therapy sessions. During a follow-up period of 14 months, neither endoscopic nor histologic evidence of recurrence has been noted. Endoscopic microwave coagulation therapy is considered to be useful in the treatment of large sessile duodenal villous adenoma, provided that repeated endoscopic follow-up surveillance is also performed.