Background: Given the rising costs of health care, policymakers are increasingly interested in identifying the inefficiencies in our health care system. The objective of this study was to determine whether the overuse and misuse of health care services in the ambulatory setting has decreased in the past decade. Methods: Cross-sectional analysis of the 1999 and 2009 National Ambulatory Medical Care Survey and the outpatient department component of the National Hospital Ambulatory Medical Care Survey, which are nationally representative annual surveys of visits to non-federally funded ambulatory care practices. We applied 22 quality indicators using a combination of current quality measures and guideline recommendations. The main outcome measures were the rates of underuse, overuse, and misuse and their 95% CIs. Results: We observed a statistically significant improvement in 6 of 9 underuse quality indicators. There was an improvement in the use of antithrombotic therapy for atrial fibrillation; the use of aspirin, (3-blockers, and statins in coronary artery disease; the use of (3-blockers in congestive heart failure; and the use of statins in diabetes mellitus. We observed an improvement in only 2 of 11 overuse quality indicators, 1 indicator became worse, and 8 did not change. There was a statistically significant decrease in the overuse of cervical cancer screening in visits for women older than 65 years and in the overuse of antibiotics in asthma exacerbations. However, there was an increase in the overuse of prostate cancer screening in men older than 74 years. Of the 2 misuse indicators, there was a decrease in the proportion of patients with a urinary tract infection who were prescribed an inappropriate antibiotic. Conclusions: We found significant improvement in the delivery of underused care but more limited changes in the reduction of inappropriate care. With the high cost of health care, these results are concerning.