Aims. To examine the level of microbial colonisation in intravenous fluids after 24 hours of use in an acute care setting to determine the necessity of changing infusate bags on a time-related basis. Background. Catheter-related bloodstream infections are a serious and life-threatening complication of intravascular devices. Colonised intravenous fluids are one potential source of infection; however, there is little published literature on incidence rates and few recent studies. Routine intravenous fluid replacement has been advocated as an infection control method, but the effectiveness of this is unknown and the optimal duration for infusate use remains uncertain. Design. Cross-sectional study over 18 months in a 257-bed teaching hospital. Methods. Infusate specimens (n = 264) were obtained from crystalloid fluids that had been used for 24 hours or more. Microbiological culture and sensitivity testing was performed and infusate-related bloodstream infection (IRBSI) rates were recorded. Sample testing of previously unopened intravenous solutions acted as a control. Results. The infusate colonisation rate was 0紥, or 0簹 per 1000 infusion hours. The only isolated organism was coagulase-negative Staphylococcus. Infusions had been in use for 24-185 hours (1-8 days). There was no difference in median duration of use for colonised (35簠hours) and sterile (34簠hours) specimens (Mann-Whitney test, p = 0繹). There were no cases of IRBSI. Conclusion. The incidence of intravenous fluid colonisation and the risk of related bloodstream infection are low even after several days of infusate use. Current practice appears to successfully maintain the sterility of intravenous fluids. Relevance to clinical practice. Routine replacement of intravenous fluids continues in many settings, often 24 hourly, in the belief that this prevents infection. We found no relationship between duration of use and colonisation and routine replacement may be unnecessary. Further research is needed to investigate the effectiveness of routinely replacing intravenous fluids at set time points to prevent colonisation and infection.