AbstractThe coronary sinus (CS) is a small tubular structure just above the posterior left atrioventricular junction. The CS can be imaged in several different echocardiographic views. Using zoom M‐mode recordings of the CS in apical two‐chamber view, CS caliber can be sharply imaged and easily measured during different phases of the cardiac cycle. We have recently shown that the CS narrows during atrial contraction in persons with sinus rhythm, but does not narrow at all if atrial fibrillation is present. Attenuation of CS narrowing occurs in patients with congestive heart failure and inferior vena cava plethora. Maximal CS caliber occurs during ventricular systole. Patients with poor left ventricular systolic function show mild CS dilatation. Greater CS dilatation is present in patients with persistent left superior vena cava, and huge dilatation when this anomaly is accompanied by absence of a right superior vena cava. Injection of sonicated saline into a left and then a right arm vein is diagnostically useful in confirming these two venous anomalies. Pulsed‐wave Doppler of the CS can be recorded in the parasternal right heart inflow view. From this and from the CS cross‐section area it may be possible to estimate coronary blood flow.