Between 1981 and 1990, 714 patients underwent 756 percutaneous transluminal coronary angioplasty (PTCA) procedures. A total of 52 patients (6.9%) had major in-hospital complications: 5 patients (0.66%) died, Q-wave or non Q-wave myocardial infarction were observed in 13 patients (1.66%) during procedure and in 8 (1%) outside the catheterization laboratory, before discharge. Because of periprocedural occlusion 11 patients (1.5%) were managed with bypass surgery, 8 (1%) had a transient occlusion that was reopened with PTCA. 21 patients (2.8%) were not ++re-dilated but managed pharmacologically. Dissection, intracoronary thrombus and previous thrombolytic treatment were often associated with occlusion. The risk of dissection was related to lesion morphology. Long-(more than 1 cm) lesion, eccentric stenosis and tortuosity of the vessel segment undergoing dilatation were risk factors for occlusive dissection. There was a high risk of side branch occlusion if its take-off was narrowed and side branch originated from the target lesion. One of the most important risk predictors is the amount of jeopardized myocardium supplied by the target coronary artery. Acute closure of an artery supplying large amount of myocardium may cause abrupt hemodynamic collapse. Hypotension secondary to the artery occlusion may cause a decrease of the flow in the other coronary arteries, leading to cardiogenic shock. Although it is important to note that patients with unstable angina, intracoronary thrombus, long and complex lesion, severe multivessel disease and compromised left ventricular function are at higher risk of acute complication, PTCA is a relatively safe procedure.(ABSTRACT TRUNCATED AT 250 WORDS)