This case reports muellerianosis of the urinary bladder, showing glandular lesions made up of endocervical type glands, in a 36-yr-old woman. The patient presented with lower abdominal discomfort and pain on voiding. The patient had undergone two Cesarean sections 5 yr and 3 yr earlier. On a pelvic ultrasonography, a well-circumscribed mass, 2.2×0.8 cm in dimension, was found with luminal polypoid projection in the posterior wall of the urinary bladder. The patient had a transurethral resection of the bladder mass. Histologically, the tumor was composed of irregularly shaped glands lined by endocervical mucous epithelium in the muscularis propria of the urinary bladder. Some glands exhibited cystic dilatation and contained mucinous secretions. The glands elicited no desmoplastic tissue reaction. The intraluminal mucin often contained polymorphonuclear leukocytes. The glands were mostly lined by tall columnar and bland looking mucous cells with mucin secretion. The ciliated cells were rarely observed. No endometrial component is noted. It would be appropriate to designate this lesion as "Muellerianosis of the urinary bladder, endocervicosis type". Awareness of the lesion and attention to its typical histologic features should facilitate its crucial distinction from adenocarcinoma.