ABSTRACT: BACKGROUND: Surveillance of healthcare-associated infections (HAI) is a valuable measure to decrease infection rates. Across Europe, inter-country comparisons of HAI rates seem limited because some countries use US definitions from the US Centers for Disease Control and Prevention (CDC/NHSN) while other countries use European definitions from the Hospitals in Europe Link for Infection Control through Surveillance (HELICS/IPSE) project. In this study, we analyzed the concordance between US and European definitions of HAI. METHODS: An international working group of experts from seven European countries was set up to identify differences between US and European definitions and then conduct surveillance using both sets of definitions during a three-month period (March 1st -May 31st, 2010). Concordance between case definitions was estimated with Cohen[RIGHT SINGLE QUOTATION MARK]s kappa statistic (kappa). RESULTS: Differences in HAI definitions were found for bloodstream infection (BSI), pneumonia (PN), urinary tract infection (UTI) and the two key terms [LEFT DOUBLE QUOTATION MARK]intensive care unit (ICU)-acquired infection[RIGHT DOUBLE QUOTATION MARK] and [LEFT DOUBLE QUOTATION MARK]mechanical ventilation[RIGHT DOUBLE QUOTATION MARK]. Concordance was analyzed for these definitions and key terms with the exception of UTI. Surveillance was performed in 47 ICUs and 6,506 patients were assessed. One hundred and eighty PN and 123 BSI cases were identified. When all PN cases were considered, concordance for PN was kappa[THIN SPACE]=[THIN SPACE]0.99 [CI 95%: 0.98-1.00]. When PN cases were divided into subgroups, concordance was kappa[THIN SPACE]=[THIN SPACE]0.90 (CI95%: 0.86-0.94) for clinically defined PN and kappa[THIN SPACE]=[THIN SPACE]0.72 (CI95%: 0.63-0.82) for microbiologically defined PN. Concordance for BSI was kappa[THIN SPACE]=[THIN SPACE]0.73 [CI95%: 0.66-0.80]. However, BSI cases secondary to another infection site (42% of all BSI cases) are excluded when using US definitions and concordance for BSI was kappa[THIN SPACE]=[THIN SPACE]1.00 when only primary BSI cases, i.e. Europe-defined BSI with [RIGHT DOUBLE QUOTATION MARK]catheter[RIGHT DOUBLE QUOTATION MARK] or [LEFT DOUBLE QUOTATION MARK]unknown[RIGHT DOUBLE QUOTATION MARK] origin and US-defined laboratory-confirmed BSI (LCBI), were considered. CONCLUSIONS: Our study showed an excellent concordance between US and European definitions of PN and primary BSI. PN and primary BSI rates of countries using either US or European definitions can be compared if the points highlighted in this study are taken into account