Focal cerebral arteriopathy of childhood (FCA)—one of the most common causes of arterial ischemic stroke in a healthy child—is an acute, monophasic, and presumed inflammatory arteriopathy of the distal internal carotid artery and its proximal branches. It has an aggressive natural history, typically progressing from mild arterial irregularity at presentation to high-grade stenosis within days. Greater severity of the arteriopathy correlates with larger infarct size and poorer neurological outcomes. The time interval from presentation to maximal severity represents a window of opportunity to intervene and improve outcomes. Current management includes aspirin, supportive care, and high-dose corticosteroids despite the absence of efficacy data. A Delphi consensus identified a clinical trial of corticosteroids for FCA as the highest research priority amongst international pediatric stroke neurologists. Surveys of U.S. pediatric stroke investigators also indicate an unwillingness to randomize children with FCA to “no steroids,” making a traditional randomized placebo- controlled trial infeasible. The most pressing clinical question is whether to treat all children with suspected FCA immediately or wait and treat only the subset that demonstrate the disease progression characteristic of FCA. Immediate treatment has the potential advantage of preventing FCA progression, but the disadvantage of diagnostic uncertainty at initial presentation, leading to unnecessary steroid exposure in children with other stroke etiologies. Clinicians also lack safety data needed for corticosteroid risk/benefit discussions with families of children with FCA. The primary aim of the Focal Cerebral Arteriopathy Steroid (FOCAS) trial is to compare the effectiveness of two strategies for treating suspected FCA with corticosteroids: (Strategy A) immediate treatment of all patients, versus (Strategy B) selective treatment of only those that demonstrate disease progression confirming the FCA diagnosis. The secondary aim is to determine the safety and tolerability of corticosteroid therapy in the setting of FCA and acute ischemic brain injury. Using a comparative-effectiveness trial design, FOCAS will prospectively enroll 80 children with suspected FCA presenting with arterial ischemic stroke or transient ischemic attack at 25 centers over 5.5 years and randomize them 1:1 to Strategy A or B. The primary endpoint will be an imaging outcome: change in FCA severity score from baseline to 1 month, measured by blinded central neuroradiologists comparing MRAs performed on the same scanner. Infarct volume at 1-month and neurological outcome at 6-months will be secondary endpoints. FOCAS safety outcomes will address clinical concerns for severe infection and hemorrhagic transformation of infarctions due to steroid-induced hypertension. The overall goal is to obtain clinically pertinent evidence that will immediately guide FCA management and help effect better outcomes for children with thi...