Haziness at sites of balloon angioplasty is believed to represent plaque fractures or platelet deposition. The etiology of haziness adjacent to coronary stents remains uncertain. This study examines the prevalence and etiology of "peri-stent" haziness following high-pressure deployment. Consecutive patients undergoing coronary stenting and intravascular ultrasound imaging were included. Haziness was defined as nonhomogeneous contrast density and/or indistinct vessel borders by consensus of 2 observers. Patients were excluded if angiography revealed an obvious cause of haziness (thrombus, dissection). Matched control segments without haziness were selected for comparison. The most diseased site within the reference segment was identified by ultrasound. Lumen and plaque areas, percent plaque area, and plaque echo density were assessed. Haziness was identified within 31 segments in 30 patients (15% of 201 angiograms examined). At hazy sites, ultrasound revealed a large percent plaque area in 15, dissections in 14, and near-normal findings in 2 segments. In the absence of dissection, percent plaque area and lumen area step-down from the stent to the diseased reference were greater than controls (percent plaque area 64 +/- 12% vs 56 +/- 10%, p = 0.04 and lumen step-down 35 +/- 20% vs 13 +/- 25%, p = 0.006). With dissections, percent plaque area and lumen step-down were not different from controls (p = 0.13 and 0.30, respectively), but underlying plaques were more frequently echolucent (64% vs 23%, p = 0.02). Thus, in this study, peri-stent haziness was evident in 15% of patients after high-pressure coronary stent deployment. Etiologies identified by intravascular ultrasound included unrecognized reference plaque and angiographically occult dissections.