PURPOSE: To evaluate angiographic variations of the circle of Willis in a normal group, and to compare thepattern of these variations between normal and aneurysm groups. MATERIALS AND METHODS: We reviewed 220 cases inwhich subjects had undergone digital subtraction cerebral angiography which showed the circle of Willis in itsentirety. One hundred patients were diagnosed as normal, and 110 had an aneurysm. We reviewed the absence orpresence and relative size of each segment, and the configuration of the distal basilar artery. Where the circlewas complete, we evaluated whether or not the circle was balanced. And we compared the pattern of these variationsbetween the two groups. RESULTS: In the normal group, the most common pattern was a small anterior communicatingartery with symmetrically large A1 segments in the anterior circulation (27%) and symmetrically small posterior communicating arteries with large P1 segments in the posterior circulation (39%). The anterior half of the circlewas complete in 90% of cases, and the posterior half in 63%. In cases in which it was incomplete, there wasagenesis of the anterior communicating artery in 9% of cases, of the A1 segment in 1%, of the posterior communicating artery in 36%, and of the P1 segment in 3%. Unilateral absence of the posterior communicating arteryand P1 segment was noted in two cases. In the distal basilar artery, symmetric cranial fusion was most common,accounting for 77% of cases. In contrast, incomplete circles with agenesis of the A1 (1% vs 12%, p<.05) or P1segment (3% vs 10%, p<.05) were more common in the aneurysm group than among normal subjects. Unbalanced typeswith a size discrepancy between A1 segments (8% vs 18%, p<.05), or posterior communicating arteries equal to orlarger than the P1 segment (27% vs 46%, p<.05) were also more common. In the distal basilar artery, symmetriccranial fusion was most common, and accounted for 69% of cases. CONCLUSION: A complete and balanced circle wasmore common in the normal group. The most common pattern was a small anterior communicating artery withsymmetrically large A1 segments and symmetrically small posterior communicating arteries with large P1 segments.In contrast, incomplete or unbalanced circles were more common in the aneurysm group.