Angioscopy of the inferior vena cava (IVC) was performed in 10 patients. Tumor thrombus extending into the IVC was suspected in six patients, and direct invasion of the IVC wall was suspected in four, based on computed tomographic and venographic findings. In each case, a fiberscope was directed to the area of interest in the IVC via a transfemoral 5-F catheter. Blood displacement by means of a saline infusion enabled angioscopic observation of the IVC wall. The neoplasms were seen clearly in all patients but one. The color and texture of the tumor thrombi surface made it easy to distinguish from the healthy IVC wall. In the four patients believed to have direct invasion of the wall, there were no angioscopic differences between the suggestive areas and adjacent areas of normal IVC wall. In these four cases, the absence of direct invasion was confirmed at surgery. Thus, angioscopy helps make an accurate nonoperative or preoperative diagnosis of tumor thrombi possible and may aid in the exclusion of direct invasion.