Medication errors are common in hospitalized patients, and are a common reason for emergency room visits. This prospective, observational study examined the frequency with which medication errors resulted in the need for hospital admission. Fully one-quarter of admissions were directly related to a medication error, most frequently an adverse drug reaction or an incorrectly selected medication. This rate is much higher than a prior study performed in cardiology patients, which found a 6% incidence of medication error–related hospitalizations. As in other studies, the medications most commonly associated with errors were cardiovascular agents and antibiotics.