A 53-year-old man presented with left hand weakness. Brain MRI revealed acute infarction in the right splenium (figure, A) of corpus callosum. Digital subtraction angiography showed a dilation with stenosis lesion in the right A2 segment (figure, B) of anterior cerebral artery (ACA). 3D High-resolution MRI (HRMRI) revealed an intimal flap and double lumen sign in the ACA (figures, C–E), confirming the diagnosis of dissecting aneurysm. Follow-up HRMRI detected the segmental luminal change from dilation to stenosis (figure, F, arrow, 3 months later). The splenium is generally supplied by a branch of the posterior cerebral artery.1 ACA dissecting aneurysm is a rare etiologic mechanism. HRMRI could help achieve a definite diagnosis of dissection.