The aim of the study was to determine whether the placement of a SmartSite antireflux valve between a bag of fluid with line giving set and an intravenous cannula decreases the flow rate. Fluid flow rates were compared in paired experiments with and without an antireflux valve using a pressure infuser or allowing fluid to flow by gravity only for four different sizes of intravenous cannulas (20-14 G). Antireflux valves significantly decreased the fluid flow rates for all cannula sizes, irrespective of whether a pressure infuser was used. The decrease in the flow rate was most marked for larger cannulas [gravity only: 14 G: 52% (95% confidence interval (CI): 37-68), 16 G: 39% (95% CI: 34-43); pressure bag: 14 G: 74% (95% CI: 70-78), 16 G: 56% (95% CI: 54-57); all P<0.001]. Where maximum fluid flow rates are desirable, clinicians should consider not using an antireflux valve.