We made continuous recordings of the X, Y and Z Frank leads in 43 patients with their first myocardial infarction admitted within 4 hours of the onset of pain. Sequential hourly analysis of the ST and QRS vector changes during the first 24 hours was performed. In short-term survivors (n = 38) mean serial changes of ST vectors showed an initial rapid decline until the 8th-10th hour, whereas QRS vector changes lasted longer and were completed within 13.5 +/- 3.6 hours (inferior infarction) and 10.2 +/- 2.4 hours (anterior infarction) respectively. The initial ST vector magnitude was significantly correlated to the subsequent cumulative QRS vector change (r = 0.82). The individual ST vector changes showed a rapid decline in 33 of 38 patients (87%), whereas in 5 patients the ST vector magnitude increased to reach its maximum after 4-6 hours. After the initial decline new increases of the ST vector magnitude was noted in 16 patients on 20 occasions. In 13 instances this was associated with recurrent pain. The spatial change of ST vector direction with reference to the initial direction was significantly greater when recurrent ST rises were accompanied by additional QRS vector changes, compared to those without associated QRS changes (P less than 0.025). The individual QRS vector slopes could be characterized as (1) monophasic (n = 21, 55%), (2) polyphasic (n = 11, 29%) and (3) irregular (n = 6, 16%). We conclude that continuous vectorcardiography is a suitable method for following ST and QRS vector changes that accompany acute myocardial infarction and that ST vector changes can be used to predict subsequent QRS vector changes.