Background: The aim of the study was to investigate the effects of a long term (1 year) hormone replacement therapy (HRT) on QT interval, QT dispersion (QTd) frequencies of arrhythmia and heart rate variability (HRV) parameters. Methods: Forty-six healthy postmenopausal women (mean age; 55.34 +/- 4.21) as a hormone replacement therapy group and 25 healthy premenopausal women (mean age; 35.36 +/- 6.06) as a control group were prospectively enrolled to the study. Hormone replacement therapy group was divided into two groups; estrogen replacement therapy (ERT) group (n = 23) and progestin-estrogen replacement therapy (PERT) group (n = 23). Standard 12 lead electrocardiograms and 24-h ambulatory Holler recording were obtained to evaluate the effects of one year of ERT and PERT on QT intervals, QTd, frequencies of arrhytmias and HRV parameters. Results: Long term use of ERT increases QT interval, QTd, in the frequencies of arrhytmia and HRV indexes of parasympathetic activity; however, the increase in frequencies of arrhythmia was not statistically significant (p > 0.05). Long term use of PERT did not effected QT interval, QTd, frequencies of ventricular arrhythmia and HRV parameters (p > 0.05). Frequency of supraventricular tachycardia increased in post-treatment PERT group was compared with pre-treatment PERT group. Conclusion: These findings supported the hypothesis that estrogen may directly modulate ventricular repolarization. But progestin do not effect the ventricular repolarization. However, these findings must be supported with a large-scale study. (c) 2003 Elsevier Ireland Ltd. All rights reserved.