Transthoracic intracardiac catheters are commonly used for monitoring and vascular access in postoperative pediatric cardiothoracic patients. Recently, their use has increased with the trend toward complete neonatal cardiac repairs. However, their complications are not well described, particularly in neonates. Therefore, we prospectively followed all PICU patients with transthoracic intracardiac catheters from 10-1-96 to 09-30-97. Catheter use, associated morbidity, necessary interventions, and risk factors associated with their complications were identified. Over this period, 526 (277 right atrial [RA], 156 left atrial [LA], 68 common atrial [CA], and 25 right ventricular or pulmonary arterial [PA]) catheters in 352 PICU patients were studied. Mean age was 23.1±45.1 months; 138 (39.2%) were < 3 months of age. The incidence of bleeding with line removal (mediastinal output > twice the average previous output) was 35.7%, and occurred more frequently with LA lines (47%, odds ratio (OR)= 2.0, P<0.05). However, interventions during line removal were required in only 7.6% (40/526) and hemodynamic compromise occurred in only 2.3% (12/526). Interventions included volume administration (24/526), transfusion (12/526), pleural drainage(3/526), and surgical removal (1/526). No associated deaths occurred. In a multivariate logistic regression analysis, age < 3 months (OR=4.74), catheter location (LA-OR=4.97; PA-OR=12.48), and platelet count < 50K(OR=8.59) were identified as risk factors associated with a need for intervention during line removal (P<0.05). Other complications included positive blood cultures (1.3%), thrombus (0.6%), and line non-function (9.9%). Prematurity was a risk factor for thrombus (1/13, 8%) and non-function (6/13, 46%) (OR=38.2) (P<0.05). The use of transthoracic intracardiac catheters in pediatric patients is safe and effective. Line non-function in premature infants is common. Young infants, patients with thrombocytopenia, or catheters in the LA or PA position have a greater need for interventions during line removal, warranting added precautions.