From 1978 to 1987, 350 infants and children had silicone central catheters placed for long-term venous access. Real time echocardiography showed a catheter tip thrombus in 16 patients, including 12 premature infants and four children. Catheters had been in place for eight to 560 days. Thirteen patients were receiving total parenteral nutrition (TPN) at the time thrombus was identified, and one patient had received TPN 2 weeks previously. Indications for echocardiography included sepsis (7), respiratory insufficiency (6), evaluation of heart disease (3), and catheter malfunction (1); several patients had multiple indications. Treatment was individualized and included atriotomy (2), systemic heparin and thrombolytic agents with or without catheter removal (8), catheter removal only (3), and observation (3). Four of eight patients treated with thrombolysis had complete clot lysis and four had partial dissolution. Two patients managed expectantly had resolution of the thrombus. None of the patients suffered further complications or died as a result of the thrombus, but ten of the 16 died 1 to 12 weeks later from their underlying disease. Forty-seven asymptomatic patients were studied by echocardiography to assess the incidence of unsuspected right atrial thrombus. Their catheters had been in place for a mean of 200 days and only 11 had been used for TPN. None of these patients had identifiable thrombus at the catheter tip. The incidence of catheter tip thrombus, which is higher than previously suspected, is related to prematurity, TPN, and continuous catheter use, but not duration of catheterization. The incidence is low in asymptomatic patients. Treatment regimen must be individualized and this series reflects a trend toward more conservative management.