Prescription of thromboprophylaxis is not a common practice in pediatric intensive care units. Most thrombi are catheter-related and asymptomatic, without causing acute complications. However, chronic complications of these (a)symptomatic catheter-related thrombi, that is, postthrombotic syndrome (PTS) and residual thrombosis have not been studied. To investigate these complications, critically ill children of 1 tertiary center with percutaneous inserted femoral central venous catheters (FCVCs) were prospectively followed. Symptomatic FCVC-thrombosis occurred in 10 of the 134 children (7.5%; 95% confidence interval [CI], 2.4-9.5). Only FCVC-infection appeared to be independently associated (P=0.001) with FCVC-thrombosis. At follow-up 2 of the 5 survivors diagnosed with symptomatic thrombosis developed mild PTS; one of them had an occluded vein on ultrasonography. A survivor without PTS had a partial occluded vein at follow-up. Asymptomatic FCVC-thrombosis occurred in 3 of the 42 children (7.1%; 95% CI, 0.0-16.7) screened by ultrasonography within 72 hours after catheter removal. At follow-up, mild PTS was present in 6 of the 33 (18.2%; 95% CI, 6.1-30.3) screened children. Partial and total vein occlusion was present in 1 (3%) and 4 (12%) children, respectively. In conclusion, children on pediatric intensive care units are at risk for (a)symptomatic FCVC-thrombosis, especially children with FCVC-infection. Chronic complications of FCVC-thrombosis are common. Therefore, thromboprophylaxis guidelines are warranted in pediatric intensive care units to minimize morbidity as a result of FCVC-thrombosis.