Background: Medication discrepancies have the potential to cause harm. Medication reconciliation by clinical pharmacists aims to prevent discrepancies and other drug-related problems. Objective: To determine how often discrepancies in the physician-acquired medication history result in discrepancies during hospitalization and at discharge. Secondary objectives were to determine the influence of clinical pharmacists’ interventions on discrepancies and to investigate possible patient-related determinants for experiencing discrepancies. Methods: This was a retrospective, single-center, cohort study of patients who were admitted to the acute geriatric department of a Belgian university hospital and followed up by clinical pharmacists between September 2009 and April 2010. Patients were limited to those 65 years or older who were taking 1 or more prescription drug. Medication reconciliation at admission, during hospitalization, and at discharge was conducted by an independent pharmacist who gathered information via chart reviews. Results: The reconciliation process at admission identified 681 discrepancies in 199 patients. Approximately 81.9% (163) of patients had at least 1 discrepancy in the physician-acquired medication history. The clinical pharmacists performed 386 interventions, which were accepted in 279 cases (72.3%). A quarter of the medication history discrepancies (165; 24.2%) resulted in discrepancies during hospitalization, mostly because the intervention was not accepted. At discharge, 278 medication history discrepancies (40.8%) resulted in discrepancies in the discharge letter, accounting for 50.2% of all 554 discrepancies identified in the discharge letters. The likelihood for experiencing discrepancies at admission increased by 47% for every additional drug listed in the medication history. Conclusions: Discrepancies in the physician-acquired medication history at admission do not always correlate with discrepancies during hospitalization because of clinical pharmacists’ interventions; however, discrepancies at admission may be associated with at least half of the discrepancies at discharge. Clinical pharmacist–conducted medication reconciliation can reduce these discrepancies, provided the erroneous information in the physician-acquired medication history is corrected and each intentional change in the medication plan is well documented during hospitalization and at discharge.