The objectives of this study were to determine the frequency and the types of errors which occur regarding the preparation and the administration of medication and to identify the main causes of these errors in a pediatric intensive care unit (PICU) at the University Hospital in Lausanne (Switzerland). In this prospective study, based on the observation of nurses' activities, the data were collected over a period of 10 weeks. The error classification was based on the American Society of Hospital Pharmacy (ASHP) definitions. The frequency of errors was calculated as the sum of all noted errors divided by the total administered drugs, plus the sum of all omitted drugs, multiplied by 100. The sum of all given doses plus all omitted doses gives the 'total opportunity for errors'. This total was 275 and the total frequency of errors was 26.9%. The most frequent errors were wrong-time errors (32.4%), wrong-administration-technique errors (32.4%) and preparation errors (23.0%). In relation with other studies conducted under comparable conditions, a lesser number of omissions and wrong-time errors were observed. On the contrary, administration-technique and dose-preparation errors were more frequent at our hospital. A program of systematic assistance and survey by professional pharmacists could improve the quality of the preparation and administration of medication in the PICU.