Discrepancy between admission and discharge medications is a well-recognized source of medication errors. This study sought to determine the factors leading to medication discrepancies, with a view toward developing better medication reconciliation efforts. Patients admitted to an inpatient general medicine service had their "gold standard" medication histories (compiled by a pharmacist) compared with admission and discharge orders. Unintentional discrepancies were documented at a rate of more than one error per patient, with most of the potentially harmful errors linked to discrepancies between the patient's preadmission and admission drug regimens. The authors found several risk factors for unintentional discrepancies, including limited health literacy and increasing complexity of medication regimens. The process of ensuring accurate medication reconciliation at admission was described in a prior review.