A descriptive study was conducted in a pediatric unit of a Brazilian hospital at São Paulo state. The purpose of this study were to classify the types of medication errors occurring in a pediatric unit, and to discuss the role of the nursing team in preventing these errors. The sample consisted of 23 medication errors that occurred during 20 days. Data was collected through direct and non-participant observation of medication preparation and administration. The most common errors detected were incorrect preparation technique (52.2%), incorrect time (34.8%) with higher frequency occurring during the morning shifts (50.0%), unauthorized drug (8.7%) and dose errors (4.3%). The pharmacological classes most involved in wrong technique were antimicrobials (66.7%), followed by analgesics (33.3%). The development of protocols for preparation and administration of medications can help the nursing professionals to provide medication safety, especially for pediatric patients, who usually receive fractionated doses.