BACKGROUND: A small number of high-need patients account for a disproportionate amount of Medicaid spending, yet typically engage little in outpatient care and have poor outcomes. OBJECTIVE: To address this issue, we developed ECHO (Extension for Community Health Outcomes) Care™, a complex care intervention in which outpatient intensivist teams (OITs) provided care to high-need high-cost (HNHC) Medicaid patients. Teams were supported using the ECHO model™, a continuing medical education approach that connects specialists with primary care providers for casebased mentoring to treat complex diseases. DESIGN: Using an interrupted time series analysis of Medicaid claims data, we measured healthcare utilization and expenditures before and after ECHO Care. PARTICIPANTS: ECHO Care served 770 patients in New Mexico between September 2013 and June 2016. Nearly all had a chronic mental illness, and over three-quarters had a chronic substance use disorder. INTERVENTION: ECHO Care patients received care from an OIT, which typically included a nurse practitioner or physician assistant, a registered nurse, a licensed mental health provider, and at least one community health worker. Teams focused on addressing patients' physical, behavioral, and social issues. MAIN MEASURES: We assessed the effect of ECHO Care on Medicaid costs and utilization (inpatient admissions, emergency department (ED) visits, other outpatient visits, and dispensed prescriptions. KEY RESULTS: ECHO Care was associated with significant changes in patients' use of the healthcare system. At 12 months post-enrollment, the odds of a patient having an inpatient admission and an ED visit were each reduced by approximately 50%, while outpatient visits and prescriptions increased by 23% and 8%, respectively. We found no significant change in overall Medicaid costs associated with ECHO Care. CONCLUSIONS: ECHO Care shifts healthcare utilization from inpatient to outpatient settings, which suggests decreased patient suffering and greater access to care, including more effective prevention and early intervention for chronic conditions.