Some bacteria have a natural tendency to adhere to available surfaces and to form biofilms. Biofilms have been demonstrated on right heart flow-directed catheters, endocardial pacemaker leads, urinary catheters, and other medical devices. In this study, we examined arterial and central venous catheters that had been in place in ICU patients between 1 and 14 days for the presence of bacterial biofilms by scanning electron microscopy, transmission electron microscopy, and a special scraping/sonication bacterial recovery technique. The data taken from these processes were compared with skin entry site swabs and blood cultures and correlated with patient data on infection, bacterial colonization, and antibiotic use. Extensive biofilm formation was demonstrated by scanning electron microscopy on all 42 arterial and 26 central venous catheters. Bacteria were seen within the biofilms on 69% (29/42) of the arterial and 88% (23/26) of the central venous catheters. These two direct methods for the detection of biofilm bacteria on the catheter surfaces demonstrated the presence of adherent organisms on 81% of devices examined. Some catheters that had been in place for only 1 day were colonized by bacterial biofilms. Biofilm scraped from catheter surfaces and dispersed by mixing and sonication yielded cultures (32% of catheters) of predominantly skin bacteria. In a few instances, the recovery of bacteria from biofilms on vascular catheters coincided with positive cultures from skin entry site swabs and/or from the blood of the patient, but there was no significant correlation between bacterial recovery from the catheter surface, entry site, and/or patients' blood. Direct microscopic examination of 68 vascular catheters that had been in place 1 to 14 days showed that most (81%) were colonized by bacteria growing in slime-enclosed biofilms. In many cases, this colonization of catheter surfaces could be confirmed by special biofilm culture recovery methods. Although the clinical importance of bacterial biofilms on catheter surfaces is speculative, their presence and potential to serve as a nidus for infection and bacteremia in critically ill immunocompromised hosts are cause for concern.