Angiograms from 238 consecutive patients who underwent percutaneous transluminal coronary angioplasty at the Mayo Clinic were reviewed to determine the presence of intracoronary thrombus before dilation. Patients with previously occluded vessels and those receiving streptokinase therapy were excluded. Intracoronary thrombus before dilation was present in 15 patients (6%); complete occlusion occurred in 11 (73%) of these during or immediately after dilation. None of these patients had angiographic evidence of major intimal dissection. In contrast, among the 223 patients in whom no intracoronary thrombus was present before dilation, complete occlusion occurred in 18 (8%) and in 12 was associated with major intimal dissection. The difference between the complete occlusion rates for patients with and without prior intracoronary thrombus was highly significant (73 versus 8%, respectively, p less than 0.001). Therefore, the presence of intracoronary thrombus identifies a group of patients who are at increased risk of developing complete occlusion during or after attempted coronary artery dilation.