Sixty-nine consecutive pulmonary artery catheters (PACs) were prospectively studied in a medical-surgical intensive care unit. Fifteen (21.7 percent) and two (2.9 percent) of the PACs were associated with colonization and bacteremia, respectively. These data represent an incidence of 4.98 and 0.66 episodes per 100 catheterization-days, respectively. Coagulase-negative staphylococci were the most common isolates. The source of the colonizing microorganism was the skin in 56 percent of cases, hubs in 28 percent, and unknown in 16 percent. From multivariate analysis, only more than 5 days of catheterization was significantly associated with a greater risk of colonization. Antimicrobial use was associated with negative cultures. The most useful method to diagnose colonization was the examination of both tip and intradermal segments. In addition, we suggest associate hub cultures when catheter-related bacteremia is suspected. These data may be useful in improving efficacy in the diagnosis and prevention of PAC-related infection.