Previous studies have shown that overt intrahepatic tumours are associated with subtle changes in liver perfusion that can be measured directly using duplex colour Doppler sonography (DCDS). This study assessed the predictive value of DCDS in the early detection of occult colorectal hepatic metastases. Hepatic arterial and portal venous blood flow was measured in 50 control subjects and 135 patients with colorectal cancer, 67 with overt liver metastases and 68 with an apparently disease-free liver on the basis of computed tomography and laparotomys findings. The Doppler perfusion index (DPI), defined as the ratio of hepatic arterial to total liver blood flow, was calculated. Clear separation of the DPI values of controls and those of patients with overt metastases was observed (P < 0·0001). Thirty-eight of the 68 patients with a disease-free liver also had an abnormally high DPI value. After 1 year of follow-up, 21 patients with an abnormally high DPI at the time of apparently curative primary resection had developed liver metastases and a further four had died without post-mortem examination. The 30 patients with normal DPI remain disease-free. The data suggest that DPI is of value in the early detection of occult liver metastases.