Formation of neoplastic thrombus in vena cava inferior (VCI) is well known in Wilms' tumour. We demonstrate a case of large neoplastic mass in the right atrium as prolongation from vena cava inferior in a 2 years and 2 months old girl with Wilms' tumour. The long preoperative chemotherapy (7 months) reduced the tumour and the thrombus. The surgical removal of the tumour and the thrombus was followed by further chemotherapy. During postoperative chemotherapy the control USG examinations showed the presence of a stable mass in a caval wall (a neoplastic mass or a scar?). The extension of the thrombus into the right atrium remains a therapeutic problem which has not been yet resolved.