Directional coronary atherectomy can cause ectasia (final area stenosis less than 0%), presumably due to an excision deeper than the angiographically "normal" arterial lumen. In a multicenter series in which quantitative coronary arteriography was performed after directional atherectomy in 382 lesions (372 patients), ectasia after atherectomy occurred in 50 (13%) lesions. By univariate analysis, ectasia was seen more often within the circumflex coronary artery (p = 0.008), in complex, probably thrombus-containing lesions (p = 0.015), and with higher device:artery ratios (p less than 0.001). Ectasia occurred less often in lesions within the right coronary artery (p = 0.008). Histologic analysis demonstrated adventitia or media, or both, in all patients with angiographic ectasia. Repeat angiography was performed in 188 of 271 eligible patients (69%) 6.1 +/- 2.4 months after atherectomy. Restenosis, defined as a follow-up area stenosis greater than or equal to 75%, was present in 50% of patients without procedural ectasia and in 70% of patients with marked ectasia (residual area stenosis less than -20%; p = 0.12). It is concluded that excision beyond the normal arterial lumen may occur after directional coronary atherectomy, related, in part, to angiographic and procedural features noted at the time of atherectomy. Restenosis tends to occur more often in patients with marked ectasia after coronary atherectomy.