Efforts to improve patient safety have largely focused on inpatient settings, despite the fact that there are an estimated one billion outpatient visits each year in the United States. We propose here to study the incidence of three types of medical errors and the underlying factors that contribute to them within the ambulatory care settings of a large, diverse integrated delivery system. Specifically, we will study: diagnostic error (delayed diagnosis of kidney disease), treatment error (the use of contraindicated medications among patients with a history of falls), and preventive error (lack of annual monitoring for patients on angiotensin-converting enzyme (ACE) inhibitors or angiotensin receptor blockers (ARBs)). There are unique challenges to identifying and intervening on care gaps in ambulatory care settings. Care is often diffuse, involving multiple providers over long time periods. There is an inherent delay in the availability of certain key pieces of information (e.g., laboratory results). Additionally, some care gaps only become identifiable weeks or months after their causes (e.g., failure to follow-up an abnormal test result is apparent only afte the usual time for follow-up has elapsed). Further, a greater number of healthy patients present with similar symptoms as patients who have serious health conditions, which may contribute to missed diagnoses or delayed follow-up care. And, patients play a greater role in care quality within ambulatory settings. Although electronic health records have been found to be insufficient for addressing care gaps themselves, they offer an efficient way to study medical errors in ambulatory settings. This is particularly true within integrated delivery systems, where processes of care across different settings and long time periods are comprehensively documented and readily linked. We propose to determine the incidence (Aim 1) and risk factors (Aim 2) for 3 types of medical errors within Kaiser Permanente Southern California (KPSC) from 2010-2015. KPSC serves a diverse population of 4 million current members, seen by over 6,000 providers in an estimated 12 million annual outpatient encounters. We will leverage KPSC's advanced electronic health record and innovative program in outpatient safety to efficiently identify cohorts of at-risk patients and to estimate the incidence of, and risk factors for, medica errors using Poisson regression. We also will generate insights into the causes of, and potential interventions for, medical errors in ambulatory care (Aim 3) through one-on-one interviews of patients and providers (n=90) and via input from an external Advisory Board (n=5). This study will generate novel information on the incidence and risk factors for 3 types of medical errors in ambulatory care settings. Our long-term goal is to contribute to meaningful patient care improvements through the use of electronic health data. This study represents a first step toward that goal, as it will generate information to inform future research and interventions to improve ambulatory care safety.