A retrospective study of 205 patients was performed to identify the risk factors associated with nosocomial bloodstream infection (BSI). The study occurred during a 5-month period in four medical-surgical intensive care units (ICUs) in Athens, Greece. Risk factors were determined using single and multivariate analyses. Thirty-five patients developed nosocomial BSI (17.1%). The incidence density (defined as the number of new cases of BSI divided by the total of patient-days in the population studied; Jarvis, 1997) of BSI was 14.3 per 1000 patient-days (total number of days that patients are in the ICU during the selected time period). A multivariate model showed that only three factors were significantly and independently responsible for nosocomial BSI: the length of ICU stay (adjusted odds ratios (AOR) 1.052, 95% confidence interval (CI) 1.018-1.087, P = 0.002); the presence of trauma at admission (AOR 2.622, 95% CI 1.074-6.404, P = 0.034); and nosocomial ventilator-associated pneumonia (AOR 6.153, 95% CI 2.305-16.422, P = 0.000). These results show that the factors that had most influence on the development of nosocomial BSI were those factors associated with the treatment received by patients during ICU stay.