To report and discuss four cases of renal cell carcinoma (RCC) in which preoperative investigations yielded contradictory results regarding the cranial extension of propagation of the tumor thrombus into the vena cava. An intraoperative ultrasound scan (IOU) was performed in all cases to identify the exact level of the tumor thrombus. We have performed an IOU of the vena cava in four patients with RCC propagation into the inferior vena cava. Preoperative investigations were performed in all patients and consisted of abdominal Ultrasound scan (USS), contrast enhanced CT scan and gadolinium enhanced MRI scan. Intraoperative ultrasound has identified correctly the cranial extension and the absence of tumor thrombus infiltration in all patients. The thrombus reached the suprahepatic vena cava in two cases and was confined to the infrahepatic vena cava in the remainder. Preoperative imaging investigation had failed to determine the correct cranial extension of the tumor thrombus in two patients.IOU is a very useful tool to accurately assess the precise extent of tumor thrombus and eventually the presence of vein wall infiltration. These data are of paramount importance to plan the optimal surgical approach. According to our experience this type of investigation identifies the cranial extent of a tumor thrombus inside the vena cava better than standard imaging techniques.