Intravascular ultrasound (IVUS) was used to study 104 lesions in 98 patients after excimer laser coronary angioplasty (ELCA). Lesion site external elastic membrane (EEM) and lumen cross-sectional areas (CSA) were measured; plaque+media (P+M = EEM - lumen) CSA and percentage of cross-sectional narrowing (CSN = P+M CSA/EEM CSA) were calculated; and the results were compared to a reference site. The lumen CSA (2.6 +/- 1.0 mm2) averaged 24% larger than the cross-sectional area of the largest laser catheter used, and 64 lesions (62%) fit the definition of arterial expansion (lesion EEM CSA > reference site EEM CSA). The residual percentage of cross-sectional narrowing averaged 83.8 +/- 8.8%. Dissections were present in 44% of lesions, and were more common in lesions with superficial calcium (59%) than in lesions with only deep calcium (31%) or no calcium (20%, P = 0.0102). Dissections of superficial calcified plaque had an unusual "shattered" or "fragmented" appearance. These findings suggest that excimer laser angioplasty causes forced vessel expansion with dissection, but limited atheroablation.