Moyamoya disease (MMD) is a rare, progressive occlusive disease characterized by bilateral internal carotid artery hypoplasia that often presents with ischemic stroke and intracerebral hemorrhage (ICH). Although MMD-related ICH is generally managed similarly to spontaneous ICH, we present a case in which standard management strategies may have led to an unprecedented devastating outcome. A 37-year-old female without any previous medical history presented with headache and right-sided weakness. A computed tomography (CT) scan revealed a large left basal ganglia ICH. Vessel imaging revealed diffuse narrowing of the entire anterior circulation with prominent leptomeningeal collaterals consistent with MMD. The patient's systolic blood pressure was kept under 140 mmHg. During the hospitalization, she became hypocarbic while being trialed on pressure support ventilation. Several hours later, she developed fixed and dilated pupils. Repeat CT head showed new diffuse cerebral edema with tonsillar herniation. Despite hyperosmolar therapy, paralytics, pentobarbital, and cerebrospinal fluid diversion, no improvement was noted. Unfortunately, brain MRI revealed multifocal brainstem infarcts with superimposed Duret hemorrhages. Herein, we report diffuse cerebral edema as a complication of MMD-related ICH. We hypothesize that disruptions of delicate cerebral autoregulatory mechanisms led to extensive hypoxic-ischemic injury. In the setting of ICH, aggressive blood pressure management coupled with relative hypocapnia may have likely caused vasoconstriction of poorly compliant arteries leading to worsened cerebral blood flow and ischemia. Therefore, because of its complex pathophysiology, strict adherence to eucapnia should be maintained in MMD-related ICH.