We report a rare case of asymptomatic aplastic or twig-like middle cerebral artery (Ap/T-MCA) with small unruptured aneurysms at the origin (A1) of the anomalous collateral artery containing plexiform (twig-like) networks and in the anterior communicating artery. In Ap/T-MCA, other cerebrovascular systems are usually considered to exhibit normal findings not associated with atherosclerosis. At first admission, atherosclerotic M1 occlusion was suspected because of the patient's risk factors and multiple stenoses in major intracranial arteries. Cerebral blood flow (CBF) studies revealed reduction of resting CBF and vascular reserve in the ipsilateral MCA territory. After 5 years, a diagnosis of Ap/T-MCA was reached based on detailed image examination and intraoperative findings during aneurysm clipping in combination with extracranial-intracranial (EC-IC) bypass. It should be noted that atherosclerosis can coexist with Ap/T-MCA, which is considered a congenital anomaly in which bleeding often occurs due to a ruptured aneurysm within the fragile collateral vessels. In previous reports, A1 aneurysms at the origins of the collateral arteries ruptured even when they were small. Aggressive radical surgery using EC-IC bypass can be considered for the treatment of unruptured aneurysms associated with Ap/T-MCA, but further research is needed. (Received September 14, 2020; Accepted November 4, 2020; Published 1 April, 2021).