In March 2007, the National Patient Safety Agency (NPSA) issued an alert regarding intravenous fluid (IVF) prescription to hospitalised infants and children, to be implemented in UK hospitals by September 2007. Previously, the most commonly used IVF (0.18% saline/4% dextrose) has been associated with iatrogenic hyponatraemia, resulting in four deaths and one near miss since 2000. The alert recommended 0.45% (or 0.9%) saline/5% dextrose as maintenance IVF and banned 0.18% saline/4% dextrose. We audited practice and outcome in children receiving maintenance IVF in June 2007 (before guideline implementation) and June 2008 (after guideline implementation). In June 2007, 44 (30%) children were prescribed IVF, six received IVF not recommended by NPSA alert 22 and one became hyponatraemic. In June 2008, 56 (30%) children received IVF; one received IVF not recommended by NPSA alert 22 and became hyponatraemic. The median change in serum sodium levels for all children who received IVF not recommended by NPSA alert 22 [-5 (-15 to 0) mmol/l] was significantly greater than those who received IVF recommended by NPSA alert 22 [0 (-13 to +7) mmol/l, p = 0.002]. In addition, there was a significant (p = 0.04) reduction in the number of children who had electrolytes checked while on IVF after implementation of the guideline. Implementation of a new IVF guideline has been associated with less use of IVF not recommended by NPSA alert 22, resulting in less serum sodium level reduction. The only children who became hyponatraemic received IVF not recommended by NPSA alert 22. Despite the NPSA alert and guideline implementation, less children had electrolyte levels checked while receiving IVF.