Diagnostic errors are widely acknowledged as a common patient safety problem, but difficulty in measuring these errors has made it challenging to quantify their impact. This study utilized a large national database of closed malpractice claims to estimate the frequency and severity of diagnostic errors. Researchers also sought to determine the types of diagnoses most vulnerable to misdiagnosis. Missed or delayed diagnoses accounted for 21% of 55,377 claims analyzed, and the majority of these cases resulted in permanent disability or death. These findings corroborate earlier research on closed malpractice claims in primary care and emergency department settings. Investigators found that three groups of diagnoses accounted for the majority of closed claims and high-severity events: vascular events (such as myocardial infarction and stroke), infections (such as sepsis), and cancer. This study represents an important step forward in identifying areas for improvement in diagnosis, but caution should be exercised in extrapolating these results, since malpractice claims only account for a small proportion of all adverse events experienced by patients. A previous PSNet perspective discussed momentum in the field of diagnostic error over the past several years.