The term carotid rete mirabile refers to an anatomic structure common in several lower mammals (e.g., swine). The blood supply for the intracranial arteries originates from branches of the external carotid artery, predominantly the ascending pharyngeal and internal maxillary arteries. In these animals the intracranial internal carotid artery forms from a dense network of numerous converging, small-caliber vessels. An analogous structure is rarely found in humans. Associated with segmental agenesis of the internal carotid artery, so-called carotid rete mirabile can be observed. In it numerous tortuous vessels with a diameter of 1-2 mm are found along the expected course of the internal carotid artery and coming from branches of the external carotid artery. These vessels converge to the intradural paraclinoid segment of the internal carotid artery, which shows a normal diameter. This rare pattern of collateral supply to the brain is illustrated here on the basis of two clinical case histories. Both patients presented with aneurysmal subarachnoid hemorrhage. In one, histological examination of a vessel biopsy revealed medial fibromuscular dysplasia. In both patients the rete mirabile was found in only one carotid system. The affected carotid canal in the skull base was hypoplastic. Human carotid rete mirabile probably has no inherent pathologic significance, but its frequent association with other intra- and extracranial vascular pathologies should be kept in mind.