The accurate diagnosis of inferior vena cava infiltration is important not only for preoperative staging but also for performing surgery for hepatic tumor involving the inferior vena cava (IVC). We investigated the diagnostic value of intracaval endovascular ultrasonography (ICEUS) in the diagnosis of invasion of the IVC by a prospective study of 30 consecutive patients with hepatic tumors with possible invasion of the IVC. The procedure was performed using an 8F, 20-MHz intravascular ultrasonography (US) from the right femoral vein. Twenty-nine of the 30 patients underwent surgery, and the tumor was resected in 25 cases, including 3 cases of combined resection of the IVC. The diagnostic criterion for IVC infiltration with ICEUS was obliteration of a single echogenic layer of the IVC wall or intracaval tumor mass. The results of ICEUS were compared with those of computed tomography (CT), cavography, conventional US, and intraoperative US. Vascular invasion was identified in 7 of 30 cases and was subsequently confirmed by pathological examination of four resected specimens, including one autopsy and two operative findings. The sensitivity, specificity, and overall accuracy of ICEUS for the diagnosis of the IVC invasion was 100%, 95.8%, and 96.7%, respectively, against 85.7%, 60.8%, and 66.7% for CT, and 71.4%, 73.9%, and 73.3%, respectively, for cavography. The sensitivity, specificity, and overall accuracy of conventional US was 87.5%, 59%, and 66.7%, respectively; those of intraoperative US were 100%, 81.8%, and 86.2%, respectively. ICEUS is a useful technique that can precisely evaluate the IVC for possible hepatic tumor invasion.