Environmental and genetic risk factors interact to cause venous thromboembolism. Renal vein thrombosis in the newborn has been frequently associated with "risk factors" as catheters, surgery or trauma, but it has also been demonstrated a pathogenetic role of genetic prothrombotic risk factors, i.e. activated protein C resistance and FV Leiden. The treatment of neonatal venous thrombosis varies worldwide and different approaches have been proposed. We present a case of renal vein thrombosis in a female newborn with normal plasma levels of protein C, protein S and antithrombin III, but with her genotype characterized by the presence of three prothrombotic risk factors: factor V Leiden, methylentetrahydrofolate reductase and platelet glycoprotein IIIa polymorphisms. The treatment with recombinant tissue plasminogen determined complete thrombus dissolution.