Communication between coronary arteries and large arteries or heart cavities in the form of fistula is a rare anomaly occurring in about 0.1-1.0% of coronarographed patients. Our case report describes a 65-year-old patient admitted to our department for suspicious left-right short circuit. Transesophageal echocardiography was performed and revealed communication between the left coronary artery trunk and the coronary sinus. Coronary angiography and subsequent CT coronarography confirmed the finding of a gigantic fistula between the ACS strain and the coronary sinus. Due to the extent of the short circuit, cardiac surgery was indicated - fistula ligation was performed, without subsequent revascularization. In the following course, the patient's subjective condition improved whereas dyspnea and pulmonary hypertension decreased.