We report a case study of a mature primary care–based accountable care organization that is both a health plan and a network of medical homes. Over 20 years, WellMed Inc (San Antonio, Texas) implemented many patient-centered services, experimenting to find which belong within clinics and which operate best as system functions. The adjusted mortality rate is half that of the state for people older than 65 years. Hospitalization and readmission rates and emergency department visits have not changed over time, but preventive services have improved. Phased implementation across the network makes it difficult to link improvements to specific processes but they seem to have improved outcomes collectively.