UNLABELLED: Little is known about the impact of accountable care organizations (ACO) on hospitalized heart failure (HF) patients, a high-cost and high-risk population. OBJECTIVE: We linked Medicare fee-for-service claims from 2013 to 2015 with data from American Heart Association Get With The Guidelines-HF registry to compare HF care, post-discharge outcomes, and total annual Medicare spending by ACO status at discharge. METHODS: Using adjusted Cox models and accounting for competing risks of death, we compared all-cause mortality and readmission at 1 year by ACO status with reporting of hazard ratios (HR) and 99% confidence intervals (CI). RESULTS: The study included 45,259 HF patients from 300 hospitals, with 21.1% assigned to an ACO. Patient characteristics were similar between the two groups with a few exceptions. The ACO patients lived in geographic areas with higher median income ($54400 [IQR $48600-65900] vs $52300 [$45900-61200], P < .0001). Compliance with four HF-specific quality measures was modestly higher in the ACO group (80% vs 76%, P < .0001). In adjusted analysis, ACO status was associated with similar all-cause readmission (HR: 1.03; 99% CI: 0.99, 1.07) but lower risk of 1-year mortality (HR: 0.85; 99% CI: 0.85, 0.90) compared with non-ACO status. Median Medicare spending in the calendar year of hospitalization was similar (ACO $42,737 [IQR $23,011-72,667] vs non-ACO $42,586 [$22,896-72,518], P = 0.06). CONCLUSIONS: Among Medicare patients hospitalized for HF, participation in an ACO was associated with similar rates of all-cause readmission and no associated cost reductions compared with non-ACO status. There was a lower risk of 1-year mortality associated with ACO participation, which warrants further evaluation.