BACKGROUND Preparing prescribed drugs in unit dose systems for often multimorbid patients in nursing homes is a responsible and difficult task for the nursing personnel. The aim of this study was to increase the knowledge on the quality level of drug supply. METHOD In a non-representative sample the local inspectorates for pharmacies in North Rhine-Westphalia compared the actually available drugs with the prescribed drugs documented in nursing homes. Participation was voluntary. RESULTS In 50 % of the investigated nursing homes no medication error could be found. 30 percent of the observed cases showed only one error. In 20 percent the individual preparation of drugs in unit dose systems should be improved. Nursing homes with specific quality standards showed the least number of errors. Oral and not documented arrangements seem to be a common cause for avoidable errors.