In the presented case report, a patient with persistent atrial fibrillation developed due to borreliosis and grade III left atrial spontaneous echo contrast (SEC) according to transesophageal echocardiography, in order to rule out intracardiac thrombosis and concomitant Lyme carditis, contrastenhanced cardiac magnetic resonance imaging was performed. Given the rapid progression of heart failure on the background of tachycardia, the lack of evidence for left atrial appendage thrombosis and myocarditis, electrical cardioversion was performed with the restoration of sinus rhythm and without thromboembolic complications during further follow-up. The issues of the clinical and prognostic significance of SEC, the potential of various diagnostic methods, and the management of such patients are discussed.