BACKGROUND: Heart rate variability (HRV) is an accepted and reliable means for assessing autonomic nervous system dysfunction. A 5-minute measurement of HRV is considered methodologically adequate. Several studies have attempted to use shorter recordings of 1-2 minutes or 10 seconds. The aim of this study was to determine the reliability of HRV parameters calculated from ultra-short electrocardiogram recordings. METHODS: Seventy healthy volunteers were recruited for the study. HRV was evaluated for 5 minutes according to accepted procedures. Thereafter, HRV parameters were recalculated from randomly selected 1-minute and 10-second intervals. The standard and ultra-short measurements were correlated using intraclass correlation coefficients. RESULTS: Good correlations between the 5-minute electrocardiograms (ECGs) and both the 1-minute and 10-second ECGs were noted for average RR interval, and root mean square of successive differences in RR intervals (RMSSD). No correlation was noted for standard deviation of the RR interval (SDNN) and several other HRV parameters. CONCLUSIONS: RMSSD, but not SDNN, seem a reliable parameter for assessing HRV from ultra-short (1 minute or 10 seconds) resting electrocardiographic recordings. Power spectral analysis and evaluation of other HRV parameters require longer recording periods. Further research is required to evaluate the importance of ultra-short RMSSD for cardiovascular risk stratification.