We report our findings associated with the differential diagnosis of infundibular dilation (ID) versus a small intracranial aneurysm using three-dimensional rotational angiography with volume rendering (3DRA + VR). Angiographic findings associated with IDs found via two-dimensional digital subtraction angiography (2D-DSA) or 3DRA + VR were reviewed for 138 consecutive patients with known or suspected aneurysms. Two readers independently evaluated the results of 2D-DSA and 3DRA + VR according to the same diagnostic criteria. We also evaluated the ability of 3D-DSA to show the spatial relation between IDs and anterior choroidal (AchA)/posterior communicating (PcomA) arteries. 2D-DSA and 3DRA + VR found 41 and 48 IDs, respectively. 2D-DSA missed five AchA and two PcomA IDs. 2D-DSA was significantly inferior to 3DRA + VR for displaying the spatial relation between IDs and AchA/PcomA (P = 0). Thus, 3DRA + VR provides more useful information for distinguishing IDs from aneurysms. The superiority of 3DRA + VR might be because of its ability to display the spatial relation between IDs and AchA/PcomA.