Indicators of the performance of clinical preventive services (CPS) have been adopted in the ambulatory setting to improve quality of care. The impact of CPS was evaluated in a network of 49 primary care practices providing care to an estimated 245,000 adults in the Dallas–Fort Worth area through a sample chart review to determine delivery of recommended evidence-based CPS combined with medical literature estimates of the effectiveness of CPS. In this population in 2005, CPS were estimated to have prevented 36 deaths and 97 incident cases of cancer; 420 coronary heart disease events (including 66 sudden deaths) and 118 strokes; 816 cases of influenza and pneumonia (including 24 hospital admissions); and 87 osteoporosis-related fractures. Thus, CPS have substantial benefits in preventing deaths and illness episodes.