The aims of this study were to compare the clinical and microbiological characteristics from patients with polymicrobial bloodstream infections (BSI) to those from patients with monomicrobial BSI and to determine their influence on the prognosis. A prospective study was conducted on 371 nosocomial BSI in an intensive care unit (ICU). Seventyfive (20.2%) of them were polymicrobial. The mean APACHE II score at the onset of bacteremia in polymicrobial and monomicrobial BSI were 17.7±6.6 and 18.9±7.5, respectively (p=0.228). Severe sepsis and septic shock were present in 68.0% and 50.6% of polymicrobial BSI and in 73.9% and 55.1% of monomicrobial BSI, respectively (p=0.298 and p=0.494, respectively). The length of stay and the length of stay post-infection were significantly longer in patients with polymicrobial BSI. APACHE II score at the onset of BSI, high-risk microorganisms, and septic shock were predictors of related mortality, but polymicrobial BSI and inadequate empirical antimicrobial treatment were not. Our findings suggest that the clinical and microbiological characteristics of polymicrobial BSI are not different from monomicrobial BSI, and polymicrobial BSI do not have any influence on the related mortality. However, they occurred in patients with a longer length of stay in the hospital and were associated with longer stays in the hospital after the episode of BSI. © Springer-Verlag 2011.