Inaccurate records of pre-admission medication exposure have been identified as a major source of medication error. Authors collected records of patients' pre-admission medications: 1) the most recent outpatient medication list ("EMR"), 2) the medication list recorded by admitting providers ("H&P"), and 3) a list generated by a medication reconciliation process conducted by nursing staff ("PAML"). Forty-eight sets of pre-admission records composed of 1087 medication entries were compared to a reference standard generated by trained study staff conducting an independent interview. Sensitivity was greatest for PAML (85%), compared to EMR (76%) and H&P (76%) sources. However, positive predictive value was greatest for the H&P source at 96% vs 88% and 91% for PAML and EMR sources respectively. Potentially harmful medication discrepancies were found within all lists. The authors concluded no single list was sufficiently accurate to avoid serious medication errors.