['We investigated the relationships between the autonomic nervous system, as assessed by heart rate variability (HRV) and levels of N-terminal Pro-B-type Natriuretic Peptide (Nt-proBNP) in patients with acute myocardial infarction (MI).', 'The mean of standard deviation of RR intervals (SDNN), the percentage of RR intervals with >50 ms variation (pNN50), square root of mean squared differences of successive RR intervals (rMSSD), and frequency domain parameters (total power (TP), high frequency and low frequency power ratio (LF/HF)) were assessed by 24 h Holter ECG monitoring. 1018 consecutive patients admitted <24 h for an acute MI were included. Plasma Nt-proBNP (Elecsys, Roche) was measured from blood samples taken on admission. The median (IQR) Nt-proBNP level was 681(159-2432) pmol/L. Patients with the highest quartile of Nt-proBNP were older, with higher rate of risk factors and lower ejection fraction. The highest Nt-proBNP quartile group had the lowest SDNN, LF/HF and total power but similar pNN50 and rMSSD levels. Nt-proBNP levels correlated negatively with SDNN (r\u200a=\u200a-0.19, p<0.001), LF/HF (r\u200a=\u200a-0.37, p<0.001), and LF (r\u200a=\u200a-0.29, p<0.001) but not HF (r\u200a=\u200a-0.043, p\u200a=\u200a0.172). Multiple regression analysis showed that plasma propeptide levels remained predictive of LF/HF (B(SE)\u200a=\u200a-0.065(0.015), p<0.001)), even after adjustment for confounders.'] Are high N-terminal pro-B-type natriuretic peptide levels associated with reduced heart rate variability in acute myocardial infarction?