Heart rate variability (HRV) is an accepted clinical tool for evaluating autonomic nervous system function and a marker of adverse cardiac outcome. Although 5 min long HRV recordings are considered methodologically acceptable, it remains impractical in most clinical settings. Also, while some ultrashort HRV (usHRV) parameters were found useful in healthy individuals, their applicability to patients with cardiovascular risk factors is largely unknown. Therefore, our goal was to evaluate the reliability of ultrashort ECG (usECG) indices for HRV among patients with hypertension. One-hundred and two patients with essential hypertension were included. HRV was recorded for 5 min in strictly monitored settings. HRV parameters from randomly chosen 1 min and 10 s series were analyzed. Excellent correlations were found between 1 min SD of RR interval (SDNN) (intraclass correlation coefficient (ICC) 0.973), 10 s SDNN (ICC 0.92) and 5 min SDNN results. An excellent correlation was also found between 1 min root mean square of successive differences in RR intervals (RMSSD) (ICC 0.992), 10 s RMSSD (ICC 0.982) and 5 min RMSSD. Logarithmic transformation of ultrashort 1 min HRV-triangular index using the natural logarithm (Ln) also had excellent correlation with 5 min measurements (ICC 0.9). Also, excellent correlations were found between 10 s and 1 min Ln(RMSSD), 10 s Ln(RMSSD) and 5 min measurements. Other HRV parameters measured from 1 min and 10 s periods showed lower correlations. In conclusion, evaluation of SDNN, RMSSD or Ln(RMSSD) from 10 s ECG recordings can be used to estimate autonomic nervous system function in patients with hypertension. These appealing markers can be readily calculated from any standard ECG tracing. The prognostic significance of ultrashort SDNN and ultrashort RMSSD in patients with cardiovascular risk factors needs to be determined in future prospective cohort studies.