Accountable Care Organizations (ACOs) are a delivery and payment model aiming to coordinate care, control costs, and improve quality. Medicare ACOs are responsible for 8 measures of preventive care quality. To create composite measures of preventive care quality and examine associations of ACO characteristics with performance. This is a cross-sectional study of Medicare Shared Savings Program and Pioneer participants. We linked quality performance to descriptive data from the National Survey of ACOs. We created composite measures using exploratory factor analysis, and used regression to assess associations with organizational characteristics. Of 252 eligible ACOs, 246 reported on preventive care quality, 177 of which completed the survey (response rate=72%). In their first year, ACOs lagged behind PPO performance on the majority of comparable measures. We identified 2 underlying factors among 8 measures and created composites for each: disease prevention, driven by vaccines and cancer screenings, and wellness screening, driven by annual health screenings. Participation in the Advanced Payment Model, having fewer specialists, and having more Medicare ACO beneficiaries per primary care provider were associated with significantly better performance on both composites. Better performance on disease prevention was also associated with inclusion of a hospital, greater electronic health record capabilities, a larger primary care workforce, and fewer minority beneficiaries. ACO preventive care quality performance is related to provider composition and benefitted by upfront investment. Vaccine and cancer screening quality performance is more dependent on organizational structure and characteristics than performance on annual wellness screenings, likely due to greater complexity in eligibility determination and service administration.