We report a case of reversible collateral flow between the right coronary artery (RCA) and the left coronary artery (LCA). A 55 year-old woman was admitted to our hospital because of chest oppression. Cardiac catheterization revealed 99% stenosis to seg. 1. We found good collateral vessels from LCA to RCA. We performed PTCA (Percutaneous Transluminal Coronary Angioplasty) on this lesion. PTCA was successful and this lesion improved (99%----40%). After PTCA, collateral vessels had disappeared. After one year, the patient had chest oppression on effort again. We performed cardiac catheterization and found 90% stenosis to seg. 6. So we performed PTCA on the new lesion. During balloon inflation, we found good collateral vessels from RCA to LCA (reversible collateral flow). We considered that reversible collateral flow was important for myocardial protection.