Introduction: It is well-known that autonomic function is impaired in panic disorder (PD). Heart rate variability (HRV) is a reliable and noninvasive method to evaluate autonomic functions. The aim of this study is to evaluate 24-hour heart rate variability (HRV) parameters in drug-free panic disorder patients. These results are compared with those obtained from healthy controls.Methods: Thirty-two (mean age 33.12±11.94 years, 20 female, 12 male) drug-free patients for at least 2 weeks, (for fluoksetin 3 weeks) diagnosed with panic disorder (PD) on the basis of DSM-IV criteria were included in the study. The control group comprised of 24 age and gender-matched healthy individuals (mean age 33.45±8.6 years, 17 female and 7 male) without any cardiac, endocrine (including diabetes), and neurological disease. Two groups were evaluated for any axis-I diagnosis with SCID-I. We used Hamilton Depression Rating Scale (HAM-D), Hamilton Anxiety Rating Scale (HAM-A), Panic Disorder Severity Scale (PDSS) and Anxiety Sensitivity Index-3 (ASI-3) tests in both groups. After the psychiatric evaluation 24 hours Holter ECG monitoring was performed in all participants for the determination of HRV including frequency and time domain parameters. Results:The time domain RMSSD and PNN50 values tended to be lower in panic patients but this difference did not reach significance (p=0.229, p=0.571). There was no significant difference in mean values for all time and frequency domain HRV measures over the total 24-hours recording between patients and controls. RMSSD, pNN50 and HF was increased to reflect the parasympathetic predominance in the sleeping period (p=0.0001) but there was no significant difference between groups. However LF value which showing sympathetic activity was increased significantly in healthy control and patients in the sleeping period (p=0.0001) but there was no significant difference between two groups (p=0.905). Correlation analysis in the panic disorder patients presented a negative correlation between disease age at onset HF value (p=0.004) and positive correlation between LF/HF value (p=0.007). When both smokers and non-smokers were compared in terms of HRV values, smoking significantly decreased HRV in both groups [SDNN (p=0,18)]. Conclusion:HRV parameters of panic disorder patients were not different from healthy controls. To better clarify the effects of panic disorder on HRV, factors effecting HRV should be minimized and more Holter ECG studies with longer records should be obtained to reflect the daily life activities.