This study was aimed at verifying the relationship between the severity illness index and the occurrence of healthcare associated infections. A prospective cohort was conducted in an intensive care unit of a university hospital, between August 2009 and February 2001. The Average Severity Index Score was used to evaluate the severity of illness. Data was collected by active surveillance and processed in SPSS. Univariate analysis was carried out, statistical signficance when p < 0.05 and confidence interval of 95%. The severity illness index was related to the occurrence of healthcare associated infection, as well as the length of stay in the intensive care unit, patient type, use of invasive procedure, antimicrobials, colonization and patient outcome (p < 0.05). The occurrence of infections was also apredictive factor for the occurrence of death (p = 0.000). These findings reinforce the idea that the occurrence of healthcare associate infections is a complex multifactorial chain, with emphasis on the clinical severity of the patient.