BACKGROUND: Alternative Payment Models (APMs) piloted by the Centers for Medicare and Medicaid Services (CMS) such as ACO Realizing Equity, Access and Community Health (REACH) seek to improve care and quality of life among seriously ill populations (SIP). Days at Home (DAH) was proposed for use in this model to evaluate organizational performance. It is important to assess the utility and feasibility of person-centered outcomes measures, such as DAH, as CMS seeks to advance care models for seriously ill beneficiaries. We leverage existing Accountable Care Organization (ACO) contracts to evaluate the feasibility of ACO-level DAH measure and examine characteristics associated with ACOs with more DAH. METHODS: We calculated DAH for Medicare fee-for-service beneficiaries aged 68 and over who were retrospectively attributed to a Medicare ACO between 2014 and 2018 and met the seriously ill criteria. We then aggregated to the ACO level DAH for each ACO's seriously ill beneficiaries and risk-adjusted this aggregated measure. Finally, we evaluated associations between risk-adjusted DAH per person-year and ACO, beneficiary, and market characteristics. RESULTS: ACOs' seriously ill beneficiaries spent an average of 349.3 risk-adjusted DAH per person-year. Risk-adjusted ACO variation, defined as the interquartile range, was 4.21 days (IQR = 347.32-351.53). Beneficiaries of ACOs are composed of a less racially diverse beneficiary cohort, opting for two-sided risk models, and operating in markets with fewer hospital and Skilled Nursing Facility beds had more DAH. CONCLUSIONS: Substantial variation across ACOs in the DAH measure for seriously ill beneficiaries suggests the measure can differentiate between high and low performing provider groups. Key to the success of the metric is accurate risk adjustment to ensure providers have adequate resources to care for seriously ill beneficiaries. Organizational factors, such as the ACO size and level of risk, are strongly associated with more days at home.