A rare vascular anomaly was found in a 21-week-old fetal brain. It consisted of an anastomotic loop between the internal carotid artery and the anterior cerebral artery. Other variations and anomalies were also found. Anomalies and variations of the circle of Willis are probably genetically determined and develop at an early embryonic stage. They are frequently found in association with other cerebrovascular diseases, such as aneurysms.