Using individual level data in the UK Biobank and genetic summary statistics from the largest relevant cohorts, Mendelian randomization analysis was conducted to explore the associations of genetically predicted CSF haptoglobin with the risk of catastrophic aSAH (i.e., fatal aSAH or non-fatal aSAH with at least one of: hemiparesis, aphasia, apraxia, or visual field defects within 7 days) and secondary traits, including stroke-related outcomes and imaging markers of brain swelling. This evidence concerns the gene HP and stroke disorder.