The prognostic information from 24-hour monitoring with on-line vectorcardiography (VCG) was assessed in 100 patients with a clinical diagnosis of unstable angina pectoris. ST change vector magnitude, ST vector magnitude and QRS vector difference were monitored. During a follow-up period of 343 ± 77 days, 7 patients died from cardiac causes and 8 patients had a nonfatal myocardial infarction (MI). Thirty patients were readmitted for unstable angina pectoris and 36 were revascularized because of medical refractory angina. Univariate predictors of cardiac death or nonfatal MI included greater age, rest pain during hospitalization, previous MI, diabetes mellitus and high incidence of supposedly ischemic transient ST and QRS vector changes. In multivariate analysis, a high incidence of transient ST (p < 0.01) and QRS (p < 0.01) vector changes provided additional prognostic information beyond that of clinical and exercise test data. In conclusion, VCG monitoring during the first 24 h of hospitalization for unstable angina pectoris identifies patients with increased risk of adverse cardiac events.