Introduction: Even though autonomic nervous system imbalance is a common finding in several diseases, these imbalances often remain untreated. The prognosis of pulmonary hypertension (PH) depends upon the functional capacity of the right ventricle. Because an autonomic nervous system imbalance occurs in PH, our goal was to investigate whether the magnitude of this imbalance might provide valuable information regarding the diagnosis, prognosis and possibly the treatment of PH. Objective: To investigate the sympathovagal balance in PH associated with right ventricular hypertrophy or right ventricular dilation. Methods: An electrocardiogram was performed in two groups of patients with PH evaluation of autonomic responses by spectral analysis: group 1- PH patients with right ventricular hypertrophy (PH-H; n= 6) and group 2- PH patients with right ventricular dilatation (PH-D; n= 9). In addition, a group of healthy subjects was also studied (CO; n= 5). Results: The autonomic balance, as determined by the ratio of the low (LF) and high frequency (HF) components (m/s2) of the electrocardiogram, was significantly impaired in the PH-D group (2±1.2) compared to the PH-H (0.5±0.4) and control (0.4±0.4) groups. In addition, there was significant variability in heart rates (m/s2) between all groups: PH-D (381±100), PH-H (1007±403) and control (1917±656). Conclusion: For the first time, a sympathetic/parasympathetic imbalance was shown to exist in PH associated with dilated cardiomyopathy but not in PH associated with hypertrophic cardiomyopathy.