The Circle of Willis was studied by Magnetic Resonance Angiography in 100 healthy subjects. In 41% of these cases, the arteries were arranged in the classically described way. In 21%, hypoplasia of the posterior communicating as. was noted, while in 13% the posterior cerebral as were found to originate from the internal carotid a.. In 9% of these cases, three anterior cerebral as. were present. In 3% the anterior communicating artery could not be identified, while the left posterior communicating artery was hypoplastic. In 2% the absence of a posterior communicating artery was associated with the origin of a posterior cerebral a. from the internal carotid. In another 2% the anterior cerebral as. were partially fused, and in yet another 2%, hypoplasia of both an anterior and a posterior cerebral a. was present. The remaining seven cases (7%), all different from each other, represented combinations of the above described variations. Statistical analysis indicated that anomalies occurred more commonly on the left than on the right side. The morphology of the Circle of Willis could not be correlated with either sex or Body Index.