Forty-two newborns were studied prospectively to determine the incidence of thrombosis due to central venous catheterization. Following Broviac catheter placement, the catheter tip, distal superior vena cava, and right atrium were evaluated by weekly two-dimensional echocardiograms. The presence of thrombosis was examined in relation to birth weight, gestational age, age and weight at the time of catheter placement, antithrombin III levels, and platelet counts. Six newborns (14%) were noted to have a thrombus by echocardiographic examination after the catheter had been in place for a median duration of 7 weeks. The infants with thrombus formation had significantly lower birth weights (887 +/- 231 v 1,409 +/- 766 g; P = .003) and gestational ages (27 +/- 2.4 v 30.3 +/- 4.3 weeks; P = .018) than those without thrombus. Their weights (757 +/- 203 v 1,832 +/- 1,098 g; P = .000) and ages (2.75 +/- 0.76 v 7.24 +/- 7.8 weeks; P = .002) at the time of catheter placement were also lower; the antithrombin III levels were lower at the time of catheter placement (0.32 +/- 0.08 v 0.06 +/- 0.31 U/mL; P = .001), but were normal for gestational and postnatal age. The presence of thrombosis was not related to the sex of the baby, the platelet count, or the duration of catheterization.