Of 523 consecutive patients undergoing elective percutaneous transluminal coronary angioplasty (PTCA) and 83 patients treated with coronary excimer laser angioplasty (ELCA), 17 (3.3%) had in-laboratory occlusion following PTCA and 25 (30%) following ELCA; they were enrolled into a prospective study. Successful management (reopened vessel, patency at repeat angiography within 24 h, no death, no myocardial infarction (MI), no emergency bypass surgery) including repeat lasing, subsequent PTCA, use of intracoronary nitroglycerin or streptokinase was achieved in 24 (96%) of the 25 patients with acute occlusion during ELCA. An anterior MI occurred in one patient of the laser group. Repeat balloon dilatation was successfully performed in seven of the 17 patients (41%) with acute closure during PTCA. Among the 10 patients with persistent occlusion after PTCA, five developed a limited myocardial infarction (35%). One patient required emergency CABG, and died peri-operatively. Severe spasm prior to occlusion defined by a new coronary flow depression without evidence of dissection or thrombus showed a significant positive association with acute occlusion during ELCA (P = 0.0008). Thus, in contrast to occlusion during PTCA, subsequent balloon dilatation was successfully performed in the majority of patients with acute occlusion during ELCA, implying that different underlying mechanisms are responsible for this complication. In this limited patient group, occlusion after excimer laser angioplasty was much more frequent than closure during PTCA, but was infrequently associated with major events such as myocardial infarction or death.