Studies in animals and humans have demonstrated the anatomic presence and functional significance of coronary collaterals. The extent of collateralization varies among species and among individuals. Collateral vessels are usually adequate for preserving resting regional and global ventricular function in the face of coronary obstruction. During stress, however, collateral supply may be inadequate. Collateral development is a time-dependent process during both the initial occlusion and following transient reflow and reclosure. Therefore when a previously collateralized coronary occlusion is recanalized and then recloses, the extent of the resulting collateral recruitment will depend, at least in part, upon the period of reflow between the two occlusions. The longer the reflow period, the less enhanced will be the collateralization. This is illustrated in the cases presented and has also been demonstrated in animal studies. The exact mechanisms for this recurrent collateral recruitment need further study.