We prospectively studied the incidence of catheter-related sepsis in 51 critically ill patients who underwent 52 arterial and 37 pulmonary artery catheterizations over a period of 3 months. Daily cultures of blood and catheter insertion site were done and the catheters were cultured semiquantitatively at the time of removal. Catheter colonization defined as growth of 15 or more colonies was observed with 9 (10%) catheters and bacteremia with 4 (4.5%) catheters. The skin cultures were positive in 56% of the colonized catheters compared with 11% of sterile catheters (p less than 0.01). The mean duration of catheterization of 3.8 days in colonized catheters was not different than 3.3 days in noncolonized catheters. Presence of concurrent infection and use of antibiotic did not change the rate of catheter colonization. Often, microorganisms other than those colonizing the catheter were recovered from blood. Femoral arterial catheterization appeared to be more often associated with colonization than radial catheters. It appears that the arterial and pulmonary artery catheter colonization occurs in about 10% of catheters and predisposes to catheter-related sepsis. Semiquantitative cultures of the catheter may aid in better documentation of catheter-induced sepsis.