To assess the extent of tumor invasion in lung cancer, my colleagues and I routinely use preoperative transesophageal ultrasonic endoscopy and intraoperative ultrasonography in addition to preoperative roentgenography, computed tomographic scanning, and other standard procedures. Both transesophageal ultrasonic endoscopy and intraoperative ultrasonography allow for a real-time assessment of the extent to which the lung cancer has invaded adjacent organs and are useful in determining the operability and safety margin of the involved organ or organs. We found intraoperative ultrasonography to be more accurate than transesophageal ultrasonic endoscopy, because intraoperative ultrasonography can be done at any time during the operation, as needed, and the probe can be directly applied to the desired location from a variety of angles. In contrast, when transesophageal ultrasonic endoscopy is used, the presence of air in the lung tissue can interfere with an accurate evaluation of some aspects of the tumor. Our results indicate that the sensitivity of transesophageal ultrasonic endoscopy and intraoperative ultrasonography is 68.4% and 100%, respectively, and the specificity is 81.3% for transesophageal ultrasonic endoscopy and 95.5% for intraoperative ultrasonography.