We report two pediatric cases of superior vena cava thrombosis (VTE) in patients treated for primary mediastinal large B-cell lymphoma (PMBCL). PMBCL is a rare entity in children and adolescents and no thrombosis has been described in this population. Thrombosis in lymphoma is frequently asymptomatic, detected as an incidental finding in the first months following diagnosis. The thrombosis mechanisms are often multifactorial based on veinous compression by the mass, elevated risk of thrombosis in neoplasia, and/or presence of a central catheter. The risk factors of venous thromboembolism (VTE) in lymphoma are high-grade lymphoma, comorbidities, central nervous system lymphoma, and mediastinal mass. Because thrombosis has an impact on prognosis and treatment, it seems important to improve knowledge in order to improve the diagnosis and prevention of thrombosis in lymphoma.