Approximately 20-25% of colorectal cancers have hepatic metastases at the time of operation and occult liver secondaries appear in 10-30% of curatively operated cases. Intraoperative liver ultrasonography has been reported to be the most accurate method for detecting colorectal metastases. A consecutive series of 119 colorectal cancer patients was studied by routine extracorporeal preoperative ultrasound (EUS), inspection and palpation of the liver at laparotomy and intraoperative liver ultrasound (IUS). 19 patients had liver metastases at the time of surgery. In eight, diagnosis was by EUS. Inspection and palpation yielded a further seven, and IUS alone a further four. Additional lesions were detected in 3 patients after preoperative ultrasound and in 6 patients following intraoperative palpation of the liver. In a further 4 cases IUS demonstrated additional metastases. Follow up for a median 38 months (12-59) was by clinical examination and six monthly liver ultrasound. During this time 8 patients, who were thought to have a clear liver, developed hepatic metastases. The mean time from surgery to the detection of occult hepatic metastases was 14.7 (8-26) months and 4 of these appeared in the second year. Intraoperative ultrasonography is a sensitive and useful method in detecting liver metastases, and may improve clinical staging and the selection of patients for further therapy. However some occult hepatic metastases will remain undetected.