This study was designed to assess the risk of colonization and biofilm formation of central venous catheters left in situ for seven days vs those changed over a guidewire at three days and removed at seven days. Colonization was determined using scanning and transmission electron microscopy and compared to a special scraping/sonication culture method. Thirty-one catheters were examined, and no difference was found between catheters left in situ (9 of 16 colonized) and those changed over a guidewire (11 of 15 colonized). Colonization rates rose significantly from 4 of 15 catheters at the time of guidewire change to 11 of 15 at 7 days (p less than 0.001). Of the catheters defined as colonized by SEM, the special culture technique showed bacterial growth in only 35 percent, making a negative culture result of dubious value in ruling out catheter colonization. No beneficial effect of guidewire changes in reducing colonization could be demonstrated.