Making an accurate diagnosis in the outpatient setting can be challenging. A prior survey of primary care physicians found that 5% of patients seen in a typical office practice were considered diagnostic challenges, due to patient factors (such as confusing or unusual symptoms) but also system factors (such as production pressures or lack of access to specialists). This study used previously developed trigger tools to screen the electronic medical record to identify cases of diagnostic error in primary care and characterized the type of errors and their underlying causes. As in prior research, most errors were thought to be multifactorial, but often related to faulty data gathering or data synthesis. Recent reviews have examined the evidence supporting cognitive and system-level interventions to minimize diagnostic errors. An AHRQ WebM&M commentary examines the role of faulty data synthesis in a delayed diagnosis of peripheral vascular disease.