Clinical cardiac procedures such as electrophysiology studies, catheter ablation of arrhythmias, retrograde cardioplegia delivery, cardiac resynchronization therapy and, more recently, percutaneous mitral annuloplasty, involve cannulation of the coronary sinus (CS). The presence of a membrane closing the orifice of the CS may cause difficulties during these interventions. Thus, detailed knowledge of the variations and anomalies of the valve of the CS, or the Thebesian valve, now has practical significance. To improve our understanding of this structure, classic anatomical dissection of 50 hearts from dissection room cadavers was performed. A Thebesian valve was present in the overwhelming majority (88%) of cases. Its morphology varied widely, from a few small strands of tissue, to a membrane covering more than half the CS ostium. A significant number (20%) of valves occluded >65% of the ostium, making them “potential complicating factors” in cannulation of the CS. An understanding of these anatomical variations may help in identifying and overcoming potential difficulties during clinical cardiac interventions. Clin. Anat. 25:379–385, 2012. © 2011 Wiley Periodicals, Inc.