OBJECTIVE To evaluate the influence of infarct related artery lesion morphology on the exercise stress test performed after a first myocardial infarction. METHODS We reviewed coronary angiography and the exercise stress test performed before discharge from hospital in 105 consecutive patients (91.4% male, mean age 49.6 +/- 9.2 years) with first acute myocardial infarction. Complex coronary lesions were defined by the presence of one of the following characteristics: ulcers, thrombus, shoulders, irregularities and eccentricity. According to either the existence or absence of complex coronary lesions, two groups were considered: Group I--42 patients with complex coronary lesions, and Group II--47 patients without these characteristics in coronary angiogram. Sixteen pts (14%) were excluded because the infarct related artery was occluded or existence of complex lesions in other coronary artery not related to the infarct. Left ventricular systolic function was analyzed using the "CASS score". Exercise stress test performed between the 10th and the 15th day after myocardial infarction, using Bruce protocol, were reviewed. The following parameters were analyzed: exercise time, number of metabolic equivalent units (METS), maximal heart rate attained, double product variation and number of patients with significant ST segment depression and/or angina. RESULTS No statistically significant differences between the two groups were obtained as far as age, sex, left ventricular function, number of diseased vessels and lesions severity. From the analysis of ergometric parameters we did not find any difference between the two groups of patients about exercise time (Group A--8.37 +/- 2.6 versus (vs) Group B 8.38 +/- 3.18), METS (Group A--8.22 +/- 2.87 vs Group B--8.13 +/- 2.97), maximal heart rate (Group A--88.2% vs Group B--87.5%) and double product variation (Group A--14547 +/- 5492 vs Group B--14553 +/- 5387). However, the number of ischemic response (defined by usual criteria of St-segment depression and/or angina) was significantly greater in patients with complex coronary lesions (ST--segment depression: Group A--26 pts vs Group B--19 pts, p < 0.05/Angina: Group A--16 pts vs Group B--4 pts, p < 0.001). Thus, complex coronary lesions are related to a high incidence of residual ischemic phenomena detected by electrocardiographic exercise stress test performed on predischarge period of acute myocardial infarction. Further studies will be necessary to show the prognostic value of particular angiographic characteristics found in coronary plaques.