Invasive hemodynamic procedures are increasingly used for monitoring critically ill and injured patients. The results of a prospective study on complications of 210 vascular catheterizations in 116 critically ill patients are reported. A total of 80 central venous (CV), 71 pulmonary artery (PA), and 59 arterial (A) catheterizations were performed during an interval of 6 months. Catheters were inserted by percutaneous techniques for all but seven of the procedures. CV catheters were associated with complications in 3.7%. However, complications were encountered in 10% of the PA catheterizations and 13.5% of systemic A catheterization. It appears that in a group of patients with observed hospital mortality of 36%, invasive monitoring is associated with a significant complication rate.