Intravascular ultrasound (IVUS) evaluation was performed in 33 lesions with sirolimus-eluting stent (SES) failure: 4 thromboses, 26 in-stent restenoses (including 6 edge stenoses), 4 new stenoses >5 mm proximal to the stent, and 1 patient with no evidence of the implanted SES (presumably because of embolization). A minimum stent area <5.0 mm(2) (stent underexpansion) was observed in 67% of all SES failures (in particular, 67% of intrastent restenosis); negative remodeling was observed in 4 of 6 stent edge restenoses, and new lesions were secondary to an increase in plaque area.