From 1982 to 1992, 9 cases of clinical stage I serous adenocarcinoma of the endometrium were diagnosed and treated surgically in PUMC Hospital. Of these 9 patients, myometrial invasion occurred in 8 patients, lymphatic invasion in the myometrium was demonstrated in 7 patients and 6 patients were found to have extrauterine spread. The tumors studied in this paper confirmed the aggressive behavior of this particular histologic type of endometrial carcinoma. The pathological characteristics and differential diagnosis are presented.