Invasive hemodynamic monitoring is widely used in the care of critically ill children. In a prospective study, a total of 330 arterial catheters, 397 central venous catheters, and 47 pulmonary artery catheters were placed in 467 children, between February 1981 and September 1982. Complications observed included: bleeding in seven of 774 vascular catheterizations, arterial obstruction in three of 377 arterial catheterizations, and sepsis in 11 of 774 vascular catheterizations. Of the 21 complications noted, 71% occurred in children less than 5 yr of age. A total of 19 (4.1%) children suffered complications of bleeding, arterial obstruction, and/or sepsis. In contrast, the overall mortality in this group of patients was 16%. The risk of morbidity from vascular catheterization appeared justified in this group of critically ill children.