Background and Objectives:Previous studies have reported a substantial between-country variation in the case fatality rates (CFRs) of aneurysmal subarachnoid hemorrhage (SAH). However, contrary to comparisons among countries, nationwide comparisons within countries that focus on populations with equal access to healthcare and include out-of-hospital deaths in analyses are lacking. Thus, we aimed to investigate whether the SAH CFRs vary between geographic regions within Finland.Methods:We identified all hospitalized and non-hospitalized (sudden-death) aneurysmal SAH cases in Finland during 1998-2017 via two externally validated nationwide registers. According to the municipality of residence, we divided the SAH cases into five geographic regions: Southern, Central, Western, Northern, and Eastern Finland, each served by a University Central Hospital with a neurosurgical service. In addition to overall 30-day CFRs, we computed sudden death rates and 30-day CFRs after hospitalization for each region. Using logistic and Poisson regression models, we calculated regional age-, sex-, and year-adjusted odds ratios and annual percent changes with 95% confidence intervals for CFRs.Results:During 1998-2017, we identified a total of 9 443 SAH cases, of which 3 484 (36.9%) occurred in Southern Finland. In comparison to the overall 30-day CFR of Southern Finland (35.1%), the age-, sex-, and study year-adjusted odds of SAH death were 32% (16-50%) higher in Central Finland (42.7%), 39% (23-58%) higher in Eastern Finland (43.4%), and 52% (33-74%) higher in Western Finland (47.1%). The regional differences were present among both sexes, in all age groups, and in sudden death rates as well as 30-day CFRs after hospitalization. Between 1998 and 2017, the overall 30-day CFRs decreased in Central (2.4% [1.0-3.8%] per year) and Southern (1.2% [0.2-2.2%] per year) Finland, whereas CFRs remained stable in the other regions. In the last four years of the study period (2014-2017), Southern Finland had the lowest 30-day CFR (16.5%) among hospitalized patients.Discussion:SAH CFRs appear to vary significantly even within a country with relatively equal access to healthcare. Future studies with detailed individual-level data are needed to explore whether health inequities explain the reported findings.