This case report is the first documentation of ultrasonographic confirmation of positioning of a Sengstaken-Blakemore balloon catheter (SBEC) in the management of massive postpartum hemorrhage. The sonongraphic position, while incorrect according to conventional use of uterine balloon catheters, raises the possibility of an alternative mechanism of action of the balloon: not uterine cavity tamponade but hydrostatic pressure directly around the uterine arteries, a mechanism akin to mechanical uterine artery embolization or ligation. If this hypothesis is correct, it not only elucidates how balloon tamponade works but also challenges current thinking on how postpartum hemorrhage should be managed effectively. Future studies are required to corroborate this hypothesis and we believe that sonographic confirmation of balloon catheter location should be an integral part of uterine balloon tamponade in women with massive primary postpartum hemorrhage. Copyright © 2008 ISUOG. Published by John Wiley & Sons, Ltd.