While transparent polyurethane dressings are increasingly used for the care of intravenous catheters, concern has recently been expressed regarding their microbiological safety. We have therefore compared the rate of intravenous catheter bacterial colonization after randomly assigning intensive care patients to transparent polyurethane (n = 21) or dry gauze (n = 20) dressings. Polyvinyl chloride catheters were inserted and maintained by the nurses. No antiseptic or antibiotic ointment was used. The two groups of patients were similar regarding risk factors for catheter colonization. Colonization rate was 48% (10/21) among patients with transparent dressings versus 10% (2/20) among patients with dry gauze dressings (p = 0.008). Colonizing bacterial species were Staphylococcus epidermidis (11 strains) and S. aureus (1 strain). No catheter-related bacteremia was observed. These data suggest that the colonization rate of intravenous catheters is increased by the use of polyurethane dressings, possibly increasing the risk of septic phlebitis and bacteremia.