Accountable care organizations (ACOs) were established as part of the Affordable Care Act to reduce costs, improve the patient experience, and increase the quality of care. While previous studies have examined the quality, costs, and patient experience among ACOs, the relationship between hospitals' ACO participation and its effects on hospitals' performance have been incompletely characterized. The main purpose of this study is to measure the association between hospitals' participation in Medicare Pioneer and Shared Savings Program (SSP) ACOs and readmission rates for heart failure (HF), acute myocardial infarction (AMI), and pneumonia. We employed a cross-sectional design using hospital readmission data from Hospital Compare, hospital characteristics data from the American Hospital Association Annual Survey, and market environmental data from Area Health Resource Files. We employed a descriptive analysis and linear regressions to examine how ACO participation is associated with readmission rates in these three conditions.Overall, we found that SSP ACO participation is significantly associated with a decrease in the HF readmission rate (β = 0.320, p < .05), while Pioneer ACO participation is not associated with a decrease in the HF readmission rate. In addition, we found no evidence that Pioneer ACO or SSP ACO participation is associated with reduced readmission rates for AMI or pneumonia. This study concluded that Medicare ACO programs have limited effects on readmission rates. Policy makers should consider adjusting the accountable care model to improve the quality of care.