To determine predictors of acute coronary dissection after coronary angioplasty, we studied 170 consecutive patients who underwent arterial dilatations of 234 arteries. Coronary dissection occurred in 103 (44%) arteries. More dissections occurred in women [40/73 (55%) versus 63/161 (39%), p < 0.03] and in patients with long lesions [45/74 (61%) versus 56/158 (35%), p < 0.0005]. Balloon/arterial diameter ratio was higher in patients with dissection (1.1 ± 0.2 versus 1.0 ± 0.2, p < 0.02). Complications did not differ in patients with and without dissection except for non-Q wave myocardial infarctions which were more frequent in patients with coronary dissection [10/12 (83%) versus 2/12 (17%), p < 0.01]. Thus coronary dissection during angioplasty is relatively frequent. However, most dissections are not associated with complications. Balloon dilatation of lesions in female patients and in patients with long lesions are more likely to result in dissection.