In the present paper we studied 403 patients with different etiopathogenic and clinical forms of thyrotoxicosis: toxic multinodular goiter (36.7%), toxic adenoma (4.9%), Graves' disease (27.04%), transient thyrotoxicosis (subacute thyroiditis, painless thyroiditis, Hashitoxicosis) (21.09%), T3-thyrotoxicosis (9.42%), thyrotoxicosis factitia (0.74%). Eighty-seven patients (21.5%) had cardiac disturbances. The following arrhythmias were most common: atrial fibrillation (4.00%), ventricular premature beats (2.77%), paroxysmal supraventricular tachycardia (2.23%), atrial flutter (1.00%). Congestive heart failure occurred in 10.42% of the cases. Paroxysmal tachyarrhythmias were converted to sinus rhythm in 90% of the subjects, by a selected and sustained treatment: drug therapy (carbimazole 30-40 mg/day, Lugol solution 1/2/20, 10-15 drops/day, beta-adrenergic blockers (propranolol--60-120 mg/24 hrs), calcium channel blockers (verapamil--40-60 mg/24 hrs), cardiac glycosides (deslanosid) or DC cardioversion. In order to prevent recurrences and/or complications, drug therapy was subsequently completed with subtotal thyroidectomy or radioactive iodine (131I) therapy. Thus, we succeeded in maintaining the patients in a euthyroid state, in sinus rhythm and with an adequate cardiovascular function in 95.4% of the cases.