Background: The treatment of subclinical hyperthyroidism (SH) is still controversial since there are no huge clinical trials that tried to assess the effects of therapy of SH. Aim: We aimed to determine whether a frequency and symptoms of cardiovascular abnormalities commonly associated with hyperthyroidism will reduce after the treatment with methimazole in a cohort of patients with established and untreated SH. Methods: We prospectively evaluated 29 patients with recently diagnosed SH that were naive to any treatment directed against over production of thyroid hormones. We evaluated each patient by assesing the rate of specific symptoms, clinical examination, electrocardiography, heart ultrasonography and determination of thyroid hormones and antibodies. Results: The mean age in our sample was 49±13 years with 21 (79.32%) women. The average duration of methimazole treatment was 7±1 months. Mean values of all thyroid hormones and thyroid antibodies significantly declined after the treatment (p<0.05). A significant decrease in frequency of palpitation, irregular heart beat and dyspnoea was observed in patients after the treatment (p<0.001). There was no difference in mean values of diastolic pressure before and after the treatment, however systolic pressure and heart rate declined significantly after the treatment (p<0.0001). A clear reduction in LV mass, interventricular septum thickness and thickness of posterior wall of LV was demonstrated as a response to treatment of subclinical hyperthyroidism (p<0.001). Supraventricular premature beats and sinus tachycardia completely disappeared after the treatment (p<0.002). Although there was a clear tendency for a reduction in frequencies of paroxysmal supraventricular tachycardia, atrial fibrilation and right bundle branch block, it was not statistically significant. Conclusion: Cardiovascular abnormalities associated with SH are significantly reduced after treatment with methimazole and achieving euthyroidism. Our results indicate that treatment of this syndrome is justified as it may reverse potentially harmful effects on cardiovascular system.