Data about medication errors in outpatients are limited. Medication errors, defined as discordance between the most recent flow-sheet version, the patient's diary card or the contents of the pill container were assessed systematically after announcement in an outpatient setting of lung transplant recipients by a direct observation approach. A total of 101 patients took a median of 15 (13-17) different drugs and 31 (26-38) pills daily. A total of 2253 doses of drugs were further analyzed. A total of 152 errors were identified resulting in 303 incorrect doses. Lack of keeping a diary card was the only factor significantly associated with a higher rate of incorrect doses (regression coefficient 0.24, p = 0.02). Furthermore, a significant correlation of medication errors with clinical adverse events could be demonstrated. This study shows that medication errors are frequent in a population which has to comply with a complex drug regimen. The need for error control as a basic requirement for drug adherence must be accentuated. Mostly, medication errors seem to occur system-based. Therefore, redundant error control mechanisms and alertness of both the patient and the care-giver should be initiated systematically.