Medical errors in the outpatient setting have remained a relatively understudied aspect of patient safety. This study analyzed data from malpractice claims at four liability insurers, similar to companion studies of errors in surgical and emergency department patients, to determine the frequency and causes of missed and delayed diagnoses. Diagnostic errors resulting in patient harm occurred in 181 cases, chiefly consisting of missed or delayed diagnoses of cancer. Failure to reach a timely diagnosis was generally due to multiple process breakdowns, including failure to order an appropriate diagnostic test and inadequate follow-up planning, many of which could be ascribed to physician cognitive errors. As with prior studies using chart review, reviewer's agreement on whether an error occurred was only moderate. The authors note that due to the complexity of contributing factors to outpatient errors, simple solutions are unlikely. An accompanying editorial, available via the link below, considers the differences in the nature of errors and approaches to solving them between the inpatient and outpatient settings and calls for greater attention to tackling outpatient safety issues.