Background: Although Accountable Care Organizations (ACOs) are defined by the provision of primary care services, the relationship between the intensity of primary care and population-level utilization and costs of health care services has not been examined during early implementation of Medicare Shared Savings Program (MSSP) ACOs. Our objective was to evaluate the association between primary care focus and healthcare utilization and spending in the first performance period of the Medicare Shared Savings Program (MSSP) Accountable Care Organizations (ACOs). Methods: In this retrospective cohort study, we divided the 220 MSSP ACOs into quartiles of primary care focus based on the percentage of all ambulatory evaluation and management services delivered by a PCP (internist, family physician, or geriatrician). Using multivariable regression, we evaluated rates of utilization and spending during the initial performance period, adjusting for the percentage of non-white patients, region, number of months enrolled in the MSSP, number of beneficiary person years, percentage of dual eligible beneficiaries and percentage of beneficiaries over the age of 74. Results: The proportion of ambulatory evaluation and management services delivered by a PCP ranged from46% (highest quartile, ACOs with greatest PCP focus). ACOs in the highest quartile of PCP focus had higher adjusted rates of utilization of acute care hospital admissions (328 per 1000 person years vs 292 per 1000 person years, p = 0.01) and emergency department visits (756 vs 680 per 1000 person years, p = 0.02) compared with ACOs in the lowest quartile of PCP focus. ACOs in the highest quartile of PCP focus achieved no greater savings per beneficiary relative to their spending benchmarks ($142 above benchmark vs $87 below benchmark, p = 0.13). Conclusions: Primary care focus was not associated with increased savings or lower utilization of healthcare during the initial implementation of MSSP ACOs.