On the basis of the ample basic knowledge acquired by repeated experiments, we applied our microwave tissue coagulator in endoscopic surgery, in a total of 59 patients with benign and malignant lesions encountered during a period of two years beginning in July, 1981. Hemostasis was achieved in 96.5% of all the cases with bleeding lesions in the digestive tract. Stenosis was alleviated in 86% of the cases with esophageal or rectal stenosis. Furthermore, the technique was successfully used for hemostasis and tumor reduction in inoperable early cancer cases. Our device is unique in that the electrode is thrust into tissue, which assures us of a satisfactory result. In this point, it must be clearly distinguished from an electrocoagulator or a laser coagulator, which is associated with a risk of injuring intact tissue. The range of coagulation is adjustable by changing the length of the monopolar antenna and electric output, and by employing a coaxial cable of appropriate thickness. In conclusion, our microwave tissue coagulator can be used easily and safely in clinical endoscopic surgery.