Discrepancies in patients' medications at the time of hospital admission are common. Performed at an academic medical center, this cohort study used a pharmacist-led medication reconciliation process to determine a "gold standard" medication list for newly admitted patients, identify discrepancies between patients' medication lists and the medications ordered by admitting physicians, and investigate risk factors for preventable medication errors. More than one-third of patients had at least one discrepancy, with elderly patients and patients with more complex medication regimens being at higher risk—factors also documented in prior research. Patients who presented their own medication list or pill bottles were at reduced risk. The medication reconciliation process used in this study is available as an online toolkit.