AIM: To investigate cross contamination of blood cultures associated with multiple use venting devices, which are widely used in clinical microbiology laboratories to reduce labour costs. METHODS: Systematic analysis of 13,880 blood culture results in a large teaching hospital where multiple use venting devices were employed. RESULTS: Nine series of potential cross contamination were identified in a 12 month period. Four series involved coagulase negative staphylococci and were unlikely to represent true cross contamination. Five series involved blood cultures which had significant bacterial growth at the time of venting. CONCLUSIONS: Multiple use venting devices can be associated with cross contamination of blood cultures. This may result from contamination of the internal lumen of the venting device which is not exposed to the biocide. Medical microbiologists should consider the possibility of cross contamination associated with venting procedures when interpreting blood culture results. Further development of multiple use venting devices is required to reduce the risk of cross contamination of cultures.