A total of 213 patients with carcinoma of the colon and rectum were examined to detect liver metastases. The study compared preoperative ultrasonography and inspection and palpation of the liver during surgery with intraoperative ultrasonography. Preoperative ultrasonography, inspection and palpation identified 238 metastases in 42 patients. Intraoperative ultrasonography detected 116 previously unrecognized metastatic tumours during 40 surgical procedures (P < 0·01). High resolution intraoperative ultrasonography is safe and more accurate than preoperative imaging and surgical exploratory methods. The examination is simple to perform and success appears to be related to careful attention to detail.