The aim of the research was to study the comorbidities, myocardial structure and function parameters and clinical symptoms in patients with first-detected atrial fibrillation (FDAF) depending on functional class (FC) of heart failure (HF). Among 124 patients included into the study, 46 didn’t have signs of HF, 58 had HF of I–II FC NYHA, 20 – HF of III–IV FC NYHA. We evaluated character and severity of symptoms, risk factors, comorbidities, parameters of lipid profile, renal and thyroid function, results of echocardiography and vascular ultrasound. FDAF was associated with large prevalence of background cardiovascular diseases, diabetes mellitus and renal dysfunction. Recurrent forms of AF were more prevalent in patients without HF and with HF of I–II FC NYHA, permanent AF on background of significant structural and functional myocardial pathology – in HF of III–IV FC NYHA. The compared groups significantly differed by clinical symptoms registered during FDAF. The background treatment of HF was characterized by insufficient usage of drugs with evidence of efficacy, first of all, beta-blockers. No patients received anticoagulants at the moment of FDAF diagnosis.