Information from 24-hour monitoring with on-line vectorcardiography, starting immediately after admission, was compared with results from a predischarge exercise test 3-13 days after admission. A total of 169 patients with acute myocardial infarction and 73 patients with unstable angina pectoris were investigated. Patients were followed for 487 +/- 135 days. During the follow-up period, 19 patients (8%) died from cardiac causes and 34 (14%) were hospitalized for a myocardial infarction. The QRS vector difference (QRS-VD), ST change vector magnitude (STC-VM), ST vector magnitude (ST-VM), and ST vector leads X, Y, Z were monitored. Patients with ST depression on the exercise test showed higher occurrence of transient, supposedly ischemic, episodes of QRS-VD, STC-VM, and ST-VM than patients without ST depression. The sensitivity and specificity of identifying patients with ST depression at the exercise test were respectively, 71 and 47% for QRS-VD episodes, 58 and 56% for ST-VM episodes, and 55 and 65% for STC-VM episodes. The maximum ST depression at the exercise test was related to the maximum ST depression in vector lead X (r = .44, P < .001) and the number of STC-VM (r = .40, P < .001), ST-VM (r = .37, P < .001), and QRS-VD (r = .33, P < .001) episodes on the VCG. In multivariate analysis, maximum ST depression in vector lead X and STC-VM episodes were the best determinants for ST depression at the exercise test. In a Cox regression model, the optimal combination of exercise test data in patients who died from cardiac causes exhibited a global chi-square value of 20.0. The combination of these data and the number of STC-VM episodes increased the global chi-square value to 30.6. This study indicates that in patients with acute ischemic heart disease, early continuous vectorcardiographic monitoring may predict the results from a predischarge exercise test and also contributes independent prognostic information beyond that of exercise test data.