Endoleaks were detected with helical computed tomographic (CT) angiography in five patients after placement of an aortobiliac stent-graft. The leaks were subsequently evaluated with duplex ultrasonography (US) and, in four patients, with conventional aortography as well. CT angiography revealed a total of seven endoleaks, all of which were prospectively classified as reconstitution (type II) leaks. Duplex US demonstrated six of the seven endoleaks. At duplex US, two of the leaks were characterized as attachment-site (type I) leaks; these two diagnoses were confirmed during subsequent angiography and profoundly altered clinical care. As an adjunct to CT angiography in evaluating endoleaks, duplex US provides hemodynamic information that enables further characterization of the type of endoleak and facilitates appropriate clinical care.