Intraoperative ultrasonic examination (IUE) was performed on 118 patients who were laparotomized for hepatectomy with five types of specially designed real-time linear array transducers. Many lesions which were invisible and nonpalpable from the surface of the liver were present in 58% of patients. All these lesions were detected by IUE. Among 16 tumors in 13 patients, IUE demonstrated 11 tumor thrombi (69%). By angiography, definite and equivocal diagnosis was made only in six tumor thrombi. Systematic subsegmentectomy was performed in 41 cases. The new concept of partial hepatectomy was accomplished by IUE. IUE is an indispensable operative procedure for hepatectomy.