Phlegmasia cerulea dolens, which is the most severe clinical presentation of deep venous thrombosis and results from an almost complete occlusion of the major and collateral venous outflow routes from the extremity, is very rarely seen in children. Here we describe the treatment of an 11-year-old boy with Down syndrome who presented with thrombotic occlusion of the inferior vena cava and both iliac veins. We present a step-by-step technique of endovascular mechanical thrombectomy of these veins with the use of the Aspirex®S thrombectomy device. Endovascular treatment was followed by local intravenous thrombolysis. Because of recurrent thrombosis, which occurred 4 days later, endovascular thrombectomy and thrombolysis were performed again. Finally, the treatment resulted in complete restoration of patency of occluded veins. Except for a minor local bleeding in the area of vascular access, there were no adverse events associated with endovascular management in this paediatric patient.