Objective: The administration of i.v. fluids to children is common in hospital. There are risks associated with fluid therapy, especially iatrogenic hyponatraemia. The objective of this study was to assess the workplace practices and knowledge of tertiary hospital doctors regarding paediatric i.v. fluid prescription. Methods: This is a prospective, questionnaire-based observational study conducted at a 570-bed teaching hospital in June 2009. A convenience sample of doctors (n = 150), representing all levels of experience and all specialties that regularly prescribe paediatric i.v. fluids, were invited to participate. The main outcome measures consisted of demographical data and the ability to correctly prescribe paediatric fluids measured as 'fluid calculation', 'fluid choice' and 'total' percentage scores based on a percentage score of correctly answered questions using eight clinical scenarios. Results: One hundred and six (71%) doctors returned a completed questionnaire. The great majority of respondents had a method for calculating a fluid bolus and maintenance rates (91% and 97%, respectively). Scenarios involving infants, especially where an increased risk of antidiuretic hormone secretion was possible, were answered poorly. Senior doctors performed better than junior doctors. ED and paediatric doctors performed better than those in other specialities. Conclusions: Most doctors in this Australian tertiary hospital have a correct method for prescribing bolus and maintenance fluid rates. However, the potential for adverse events from i.v. fluid prescription remains. Further education in this area for junior doctors, introduction of standardized guidelines for fluid use and restriction of available fluid choice may reduce the risk of iatrogenic hyponatraemia in children.