Leeds LS9 7TFIt has been shown that ultrasound and static isotope imaging are relatively insensitive for the detection of lesions <2cm diameter (Bryan et al., 1977). CAT scanning, although apparently more sensitive than these 2 methods still appears to have a limitation in detecting small lesions in the liver (Scherer et al., 1978). However, since it has been established that intrahepatic primary or secondary tumours are associated with an increased hepatic arterial blood flow (Breedis & Young, 1953) and time-activity curves generated. The time of the peak of the kidney curve was used to indicate the division between arterial and portal inflow phases of the liver curve. The quality of the bolus injection and its distribution was assessed by examination of the rise time of the kidney curve and studies were rejected if this was greater than 8 sec. After 3-point smoothing of the liver curve the average slopes of the 2 consecutive 8-second sections on either side of the arterial/portal division were calculated. The first slope was taken to represent arterial inflow and the second slope was taken to represent the portal inflow. The hepatic perfusion index (HPI) was expressed as a fraction of the arterial inflow to the total hepatic inflow. Static scans were independently assessed (P.J.R.) as being indicative or non-indicative of the presence of hepatic metastases. The results of the static and dynamic studies were correlated with findings at laparotomy for the presence or absence of hepatic metastases, which where possible were measured. The sites of these metastases were also noted. Figure 1 shows the distribution of HPI values in the positive laparotomy group, in the group of patients with no liver metastases and in the control group. It can be seen that 24/25 patients who were in the laparotomy positive group (96%) had HPI values above the normal range, the upper limit of normal in this series being 0.42. One patient with massive hepatic replacement by tumour had an HPI value of 0.15 but interestingly had a positive static scan. In those patients known to have hepatic involvement, the sensitivity of static scanning was 64%. Nine patients in this group had normal scans, but all had abnormal HPI values. The data for the negative laparotomy group are also shown but no definitive statement can be made until the follow-up