A 63-year-old man presented with a 2-week history of bilateral lower-extremity edema, a 30-lb weight gain, increasing shortness of breath, and paroxysmal nocturnal dyspnea. Past medical history was significant for an initial episode of congestive heart failure in March 1995. A subsequent cardiac catheterization at that time showed a 90% occluded ostial left main coronary artery with high-grade lesions involving the mid left anterior descending (LAD) and circumflex arteries. Coronary artery bypass graft surgery was performed with a left internal mammary artery (LIMA) graft to the LAD and a saphenous vein graft to an obtuse marginal artery. The patient was referred for cardiac catheterization on this admission. An AL-1 catheter could not be successfully engaged into the left main artery and was subsequently directed toward the right coronary artery. There was no damping of the arterial waveform upon engagement. Two sets of biplane images in different projections showed this to be a 3.3-mm (by quantitative angiography) conus branch off the right coronary artery. It passed upward and over the right ventricular outflow tract to provide collateral flow to the LAD and subsequently the other vessels of the left coronary artery system. This type of collateral system is known as Vieussens' ring. Competitive flow was seen at the site of the LIMA-to-LAD anastomosis.