Relationships between hypothalamic arteries and anterior communicating artery aneurysms were examined in 34 cases treated by clipping operations, using the anterior interhemispheric approach. The directions of the aneurysmal domes and hypothalamic arteries were analyzed and divided into 2 groups. The different direction(D.D.) group involved cases in which a hypothalamic artery ran in a different direction to that of the dome of the aneurysm. The same direction (S.D.) group involved cases in which a hypothalamic artery ran parallel to the dome of the aneurysm. The D.D. group consisted of 15 cases, and the S.D. group consisted of 13 cases. In the remaining 6 cases, the hypothalamic artery was not found during the operation. In many cases of the D.D. group, the aneurysm was located anterior to the A2 portions of the bilateral anterior cerebral arteries, whereas, in all cases of the S,D. group, the aneurysm was located between or posterior to the bilateral A2 portions. We also investigated the flow direction of the anterior communicating artery in the S.D. group, The dominant At was defined as the side of internal carotid angiography in which the aneurysm was depicted on preoperative angiography. The flow direction of the anterior communicating artery was assumed to flow from the dominant A1 side to the recessive A1 side. Considering the flow direction of the anterior communicating artery, the hypothalamic artery was located downstream from the aneurysm in 9 cases and upstream in 3 cases. These results suggested that it is important to pay more attention to the downstream of the aneurysm to avoid injury of the hypothalamic artery in the S.D. group. For cases in which the hypothalamic artery was located upstream to the aneurysm, these aneurysms seemed to arise from the bifurcation of the anterior communicating artery and hypothalamic artery, and might be named hypothalamic artery aneurisms.