A bladder-flap hematoma is an unusual complication of a lower-uterine transverse cesarean section. As a result of bleeding at the incision site, a hematoma forms between the bladder and lower uterine segment. The clinical, sonographic, and surgical findings in seven patients with this complication are described. In all, sonography showed a solid or complex mass of varying sizes interposed between the posterior bladder wall and the anterior uterine wall. Differentiation of a hematoma from an infected hematoma/abscess can be difficult, but the presence of air in the mass strongly suggests the latter. Sonography can detect this complication easily and accurately, and therefore significantly focus and guide therapy.