BACKGROUND--Reduced heart rate variability has been identified as an important prognostic factor after myocardial infarction. This factor is thought to reflect an imbalance between sympathetic and parasympathetic activity, which may lead to unfavourable loading conditions and thus promote left ventricular dilatation. PATIENTS AND METHODS--298 patients in a multicentre clinical trial were randomised to captopril or placebo after a first anterior myocardial infarction. All patients were treated with streptokinase before randomisation. In the present substudy full data including heart rate variability and echocardiographic measurements were available from 80 patients. Patients were divided into two groups: those with a reduced (< or = 25) heart rate variability index and those with normal heart rate variability index (> 25). Heart rate variability was evaluated by 24 h Holter monitoring before discharge. Left ventricular volumes were assessed by echocardiography before discharge and three and 12 months after myocardial infarction. Extent of myocardial injury, severity of coronary artery disease, functional class, haemodynamic variables, and medication were also considered as possible determinants of left ventricular dilatation. RESULTS--Before discharge end systolic and end diastolic volumes were not different in the two groups. After 12 months in patients with a reduced heart rate variability, end systolic volume (mean (SD)) had increased by 6 (14) ml/m2 (P = 0.043) and end diastolic volume had increased by 8 (17) ml/m2 (P = 0.024). Left ventricular volumes were unchanged in patients with a normal heart rate variability. Also, patients with left ventricular dilatation had a larger enzymatic infarct size and higher heart rates and rate-pressure products. A reduced heart rate variability index before discharge was an independent risk factor for left ventricular dilatation during follow up. Measurement of heart rate variability after three months had no predictive value for this event. CONCLUSION--Assessment of the heart rate variability index before discharge, but not at three months, gave important additional information for identifying patients at risk of left ventricular dilatation.