OBJECTIVES: Medication error rate (TEM) is seen as the performance rate of the process leading from drug prescription to drug administration. Jury-les-Metz Psychiatric Hospital conducted a pilot study whose aim was to determine feasibility of drug error rate calculation. METHODS: TEM was calculated as the number of errors divided by the number (n) of all the doses ordered. Four different steps of the process were studied with a specific rate for each step :drug prescription error rate (TEMp), drug delivery calculation error rate (TEMr), drug missing before administration rate (TEM,) and nursing error in drug preparing rate (TEMc). RESULTS: TEMp (n = 15699) : 1.92% [1.74 - 2.10], TEMr (n = 3457) 2,7% [2.2 - 3.2], TEMm (n = 3457) : 6,9% [6.1 - 77], TEMc (n = 956). 5,9% [4.4 - 7.4]. DISCUSSION: The research topic presented here is quite new. In spite of certain limitations, the most important point is to focus attention on four specific steps and not to limit the study to the entire process. Our pilot work demonstrates that drug errors are made and must not be neglected, The TEM is not simply the sum of specific errors because of the interactions between the underlying causes of the different types of errors, each affecting the value of specific TEMs. CONCLUSION: Before to deal with quality improvement of hospital medication process, we had to prove that drug error exists. Now, to solve the problem of medication error in our hospital, we have to observe what exactly happens and to try to multidisciplinary understand the causes leading to process defects. (C) 2001, Masson, Paris.