Selective embolization of thrombotic material to the borderzone territories between major cerebral arteries was found to be the cause of watershed cerebral infarction in three reported cases. The embolic source was cardiac in two cases and carotid artery in one. The findings in these cases support the hypothesis that small thromboemboli may not be randomly distributed in the cerebral arterial supply but may be preferentially distributed to the small terminal arterial branches in the borderzones between major arterial territories. Previous reports of this phenomenon laid emphasis on the sharp angle at which small penetrating arteries branched from parent arteries as the mechanism underlying this preferential distribution. We propose that the unequal size of the branches at the majority of bifurcations of subarachnoid arteries is a more important anatomic feature leading to this pattern of infarction.