Abnormal postmenstrual uterine bleeding may occur in women with history of cesarean section. To study the etiology of this anomaly, transvaginal ultrasound (TVU) was performed during follicular phase in 20 such patients, and hysteroscopy in 7. The TVU revealed the presence of fluid in the isthmus in relation to the cesarean section scar, where a kind of pouch or bursa was observed. Cervical introduction of a catheter showed that the fluid was blood. The TVU observation was confirmed in women in whom hysteroscopy was performed. Resection of ring-shaped fibrotic tissue on the inferior part of the scar stopped abnormal bleeding. Four women were infertile and two conceived, probably because stopping postmenstrual bleeding could interfere with the quality of cervical mucus. These findings suggest that abnormal uterine bleeding in women with an abnormal uterine scar could be resolved by minimally invasive endoscopic surgery, and TVU is a valuable diagnostic tool.