We have previously studied system failures involved in medication errors using a limited number of root cause analyses as source. The aim of this study was to describe a larger number of medication errors with respect to harm, involved medicines and involved system problems – thus providing information for the development of IT-based decision support. We evaluated 3,520 medication error reports derived from 12 months of consecutive reporting from 13 hospitals in the Capital Region of Denmark. We found 0.65% errors with serious harm and 16% with moderate harm. A small number of medicines were involved in the majority of the errors. The problems in the medication error process were heterogeneous. Some were related to specific medicines and others were related to the computerized order entry system. Accordingly decision support targeted at specific medicines and improved IT systems are part of the continuing work to reduce the frequency of medication errors.