The case of a young woman who developed a traumatic distal anterior cerebral artery ("pericallosal") aneurysm from a roller-coaster ride is described. She presented with a subarachnoid hemorrhage (SAH) restricted to the interhemispheric fissure. The initial angiogram was normal but repeat angiography at 8 days revealed the aneurysm. After craniotomy and clipping the patient made a satisfactory recovery. This is the only reported case of a traumatic aneurysm arising under circumstances not usually considered as trauma. It raises questions about the pathophysiology of the formation of such aneurysms and suggests that traumatic pericallosal aneurysms should be considered in SAH of unknown etiology. This case provides further evidence that repeat angiography occasionally helps reveal an aneurysm when the initial study fails to do so.