The valve of the coronary sinus was studied in 50 hearts from dissection room cadavers. It varied from a flap that covered up to 70% of the ostium of the coronary sinus, to a few small strands of tissue. In 3 cases no valve was present. In most cases (35 out of 50), the valve covered <50% of the ostium of the coronary sinus, so it is unlikely to play an important role in preventing reflux into the coronary sinus. Microscopically, the valve was found to contain layers of myocardium, most of which disappeared in the less well formed valves, but some myocardium was persistent even in the most rudimentary valves studied. © 1994 Wiley-Liss, Inc.