Objectives. We report the use of three-dimensional (volumetric) intravascular ultrasound (IVUS) analysis to assess serial changes after directional coronary atherectomy (DCA). Background. Recent serial planar IVUS studies have described a decrease in external elastic membrane (EEM) area following catheter-based intervention as an important mechanism of late lumen renarrowing. Methods. Thirty-one patients with de nero native coronary lesions treated with DCA in the Serial Ultrasound Restenosis (SURE) Trial and in Optimal Atherectomy Restenosis Study (OARS) were enrolled in this study. Serial IVUS was performed before and after intervention and at 6 months' follow-up. In a subgroup of 18 patients from the SURE trial, IVUS was also performed at 24 h and at 1 month postintervention. Segments, 20-mm-long (200 image slices), were analyzed using a previously validated three-dimensional, computerized, automated edge-detection algorithm. The EEM, lumen, and plaque+media (P+M = EEM-lumen) volumes were calculated. Results. At follow- up, lumen volume was smaller than at postintervention (159 ± 69 mm3 vs. 179 ± 49 mm3, p = 0.0003). From postintervention to follow-up, there was a decrease in EEM volume (377 ± 107 to 352 ±. 125 mm3, p < 0.0001), but no change in P+M volume (p = 0.52). The Δ lumen volume correlated strongly with ΔEEM volume (r = 0.842, p < 0.0001), but not with AP+M volume. In the 18 patients from the SURE Trial, the decrease in lumen and EEM volumes occurred late, between 1 month and 6 months of follow-up. Conclusions. Volumetric IVUS analysis demonstrated that late lumen volume loss following DCA was a result of a decrease in EEM volume. This was a late event, occurring between 1 and 6 months' postintervention.