OBJECTIVE: The prevalence of asymptomatic catheter-related thrombosis of the upper venous system in children with cancer has not been determined. We evaluated patients with cancer and implantable central venous catheters (ports) for this complication. STUDY DESIGN: Children with cancer undergoing port removal were eligible for this study. Vessel patency was evaluated by contrast venography. We examined each child for physical stigmata of thrombosis and retrospectively assessed catheter-related mechanical difficulties and infections. RESULTS: Thirty-one ports had been placed in 24 children (aged 20 months to 18 years; median age, 9 years) with diagnoses of leukemia/lymphoma (n = 10), solid tumor (n = 12), and histiocytosis (n = 2). Venography showed abnormalities in 12 of the 24 patients. Physical examination revealed dilated superficial veins on the chest in 3 patients. Venograms showed abnormalities in all 3 children with prominent superficial thoracic veins. Nine of the 21 other patients had clinically occult central venous occlusion. CONCLUSION: Fifty percent (95% CI, 30% to 70%) of children who had implantable ports removed during or after treatment of cancer exhibited deep venous thrombosis at the site of catheter placement. Future studies should determine the contribution of inherited and other acquired risk factors for thrombosis and assess measures to prevent and/or treat catheter-related thrombosis in this population.