Routine vaginal ultrasonographic evaluation of pelvic organs was performed in asymptomatic postmenopausal women to evaluate the diagnostic significance of sonographic findings of endometrial fluid collections in the presence of a thin endometrium in postmenopausal women. The study included nine women aged 61 to 79 years who had been postmenopausal for 7 to 30 years, and in whom endometrial fluid collections were incidently demonstrated. Endometrial samplings followed the ultrasonographic examinations in all nine patients, and the correlation between the histologic report and the endometrial ultrasonographic thickness was evaluated. The diameter of the endometrial fluid collections varied from 3 to 12 mm. The thickness of the endometrium surrounding the fluid ranged from 1 to 3 mm. Of the nine endometrial samples examined, five revealed atrophic endometrium, three had insufficient material for histologic evaluation, and one specimen revealed well-differentiated adenocarcinoma. The woman in whom endometrial carcinoma was diagnosed underwent total abdominal hysterectomy with bilateral salpingo-oophorectomy and lymph node sampling. The pathologic examination confirmed the previous histologic finding, and deep penetration into the myometrium was noticed with intact lymph nodes. Endometrial fluid collections found in asymptomatic postmenopausal patients still may be associated with endometrial malignancy, even in the presence of thin endometrium.