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Clinical trials show benefit from lowering systolic blood pressure (SBP) in people ≥80 years of age, but nonrandomized epidemiological studies suggest lower SBP may be associated with higher mortality. This study aimed to evaluate associations of SBP with all-cause mortality by frailty category >80 years of age and to evaluate SBP trajectories before death.A population-based cohort study was conducted using electronic health records of 144 403 participants ≥80 years of age registered with family practices in the United Kingdom from 2001 to 2014. Participants were followed for ≤5 years. Clinical records of SBP were analyzed. Frailty status was classified using the e-Frailty Index into the categories of fit, mild, moderate, and severe. All-cause mortality was evaluated by frailty status and mean SBP in Cox proportional-hazards models. SBP trajectories were evaluated using person months as observations, with mean SBP and antihypertensive treatment status estimated for each person month. Fractional polynomial models were used to estimate SBP trajectories over 5 years before death.During follow-up, 51 808 deaths occurred. Mortality rates increased with frailty level and were greatest at SBP <110 mm Hg. In fit women, mortality was 7.7 per 100 person years at SBP 120 to 139 mm Hg, 15.2 at SBP 110 to 119 mm Hg, and 22.7 at SBP <110 mm Hg. For women with severe frailty, rates were 16.8, 25.2, and 39.6, respectively. SBP trajectories showed an accelerated decline in the last 2 years of life. The relative odds of SBP <120 mm Hg were higher in the last 3 months of life than 5 years previously in both treated (odds ratio, 6.06; 95% confidence interval, 5.40–6.81) and untreated (odds ratio, 6.31; 95% confidence interval, 5.30–7.52) patients. There was no evidence of intensification of antihypertensive therapy in the final 2 years of life.A terminal decline of SBP in the final 2 years of life suggests that nonrandomized epidemiological associations of low SBP with higher mortality may be accounted for by reverse causation if participants with lower blood pressure values are closer, on average, to the end of life.