IMPORTANCE: Widespread deficits in the quality of US health care were described over a decade ago. Since then, local, regional, and national efforts have sought to improve quality and patient experience, but there is incomplete information about whether such efforts have been successful. OBJECTIVE: To measure changes in outpatient quality and patient experience in the United States from 2002 to 2013. DESIGN, SETTING, AND PARTICIPANTS: We analyzed temporal trends from 2002 to 2013 using quality measures constructed from the Medical Expenditure Panel Survey (MEPS), a nationally representative annual survey of the US population that collects data from individual respondents as well as respondents’ clinicians, hospitals, pharmacies, and employers. Participants were noninstitutionalized US adults 18 years or older (range, 20 679-26 509 individuals each year). MEASURES: Outpatient quality measures were compiled through a structured review of prior studies and measures endorsed by national organizations. Nine clinical quality composites (5 “underuse” composites, eg, recommended medical treatment; 4 “overuse” composites, eg, avoidance of inappropriate imaging) based on 39 quality measures; an overall patient experience rating; and 2 patient experience composites (physician communication and access) based on 6 measures. RESULTS: From 2002 to 2013 (MEPS sample size, 20 679-26 509), 4 clinical quality composites improved: recommended medical treatment (from 36% to 42%; P < .01), recommended counseling (from 43% to 50%; P < .01), recommended cancer screening (from 73% to 75%; P < .01), and avoidance of inappropriate cancer screening (from 47% to 51%; P = .02). Two clinical quality composites worsened: avoidance of inappropriate medical treatments (from 92% to 89%) and avoidance of inappropriate antibiotic use (from 50% to 44%; P < .01 for both comparisons). Three clinical quality measures were unchanged: recommended diagnostic and preventive testing (76%), recommended diabetes care (68%), and inappropriate imaging avoidance (90%). The proportion of participants highly rating their care experience improved for overall care (from 72% to 77%), physician communication (from 55% to 63%), and access to care (from 48% to 58%; P < .01 for all comparisons). CONCLUSIONS AND RELEVANCE: Despite more than a decade of efforts, the clinical quality of outpatient care delivered to American adults has not consistently improved. Patient experience has improved. Deficits in care continue to pose serious hazards to the health of the American public.