We reported a case of a twenty-one-year-old man with an atrial flutter as the first manifestation of progressive cardiac conduction disease. The patient was admitted to the cardiology department due to complaints of shortness of breath and a decrease in exercise tolerance, which had happened after physical exercises (running). During ambulatory ECG monitoring persistent AFL was observed with atrial rate 262-297 bpm and ventricular rate 26-136 bpm (average 56 bpm). AV conduction was very variable – 4:1-14:1. The results of ambulatory ECG monitoring during the whole period of recording indicated signs of atrioventricular conduction disturbances. After cardioversion sinus rhythm was restored additional rhythm and conduction disorders were revealed. Ambulatory ECG monitoring was performed two weeks after the initial one, and throughout this recording were registered sinus rhythm on the background of first-degree AV block; transient Mobitz I AV block; and type 2 second-degree sinoatrial block. Trans-esophageal electrophysiology study was performed. During pharmacological denervation of the heart, signs of slowing of the atrioventricular conduction and sinus node recovery time persisted. These changes along with right bundle branch block were regarded as a progressive cardiac conduction disease with an apparently hereditary cause.