BACKGROUND: Medication errors may occur when hospital doctors are not adequately informed about a patient's prescribed drugs. METHODS: The drug lists of 103 patients who were electively admitted for coronary angiography were assessed. Discrepancies between lists noted in admission letters, patient's personal medication lists, and medication histories were analyzed. RESULTS: Patients took a mean of 5 +/- 3 drugs. Nine percent of all drugs taken were only mentioned when a systematic medication history was obtained but were not stated in admission letters or on medication lists. Only 88% of admission letters reported the patient's medication. Twenty-one percent of generics were incorrectly documented as originals in the admission letter. Less than 50% of patients taking >or= 4 drugs had a written instruction on how to take their medication. A total of 86 drugs actually taken by the patients were not identical to those listed in the referral letter or the medication list, leaving uncertainties as to how outpatient medication should be continued. Medication was modified in 25% of all patients at hospital discharge. CONCLUSIONS: Instructions for patients taking multiple drugs and information in admission letters need to be improved. These results underline the importance of medication reconciliation at hospital admission.