Despite the widely demonstrated association of reduced heart rate variability (HRV) to bad prognosis after myocardial infarction (MI), reference values for HRV parameters are still not available. The GISSI-3 Arrhythmias Substudy studied short-term HRV in a relatively unselected population of patients (324) with recent MI (13 +/- 7 days) providing the statistical description of the main time and frequency domain parameters. All HRV indices, except for the RR interval, showed a non-normal distribution generally skewed around the lowest values. Particularly, no LF power was detected in 75 patients (23%) by power spectral analysis. The absence of LF oscillation in RR spectra was associated to the lower standard deviation of normal RR intervals (SD), aging (> 65 years) and blood pressure hypertension. This result seems to indicate a paradoxical effect of sympathetic overactivity in post-MI patients.