The growth of 32 untreated colorectal liver metastases and hepatic parenchymal volume in 11 patients were studied using planimetry of computed tomographic images. The median percentage hepatic replacement (PHR) of metastases was 0.49 (range 0.09-18.2). At subsequent assessment a median of 72 (range 14-235) days later, metastases were significantly larger with a median PHR of 1.78 (range 0.10-22.9) (P < 0.001). To compare the growth of metastases with variable PHR, logarithmic changes of PHR over 100 days were studied. This parameter was shown to be reduced in metastases of larger volume. The median (interquartile range) non-neoplastic hepatic parenchymal volume was found to increase from 1839 (1647-1997) to 2200 (1590-2401) cm3 (P = 0.005) as metastatic burden increased. These findings suggest that the growth of colorectal hepatic metastases is more complex than a simple process of hepatic replacement.