The natural history of patients with colorectal hepatic metastases was studied following resection of the primary tumour. Patients were stratified according to the presence of extrahepatic disease and the percentage hepatic replacement (PHR) measured by planimetry of computed tomographic images. The median(semi-interquartile range (s.i.r.)) PHR of patients without extrahepatic disease was 15.0(10.5) and for those with extrahepatic disease 5.4(2.2) (P less than 0.02). The median(s.i.r.) PHR of all patients was 8.7(8.5). The median(s.i.r.) survival of all patients was 125(85.5) days, 92(114) days for patients with extrahepatic disease and 175(133.5) days for patients with disease confined to the liver (P less than 0.02). A linear correlation between the natural logarithm of PHR and survival was demonstrated (p = -0.76, P less than 0.001) for patients without extrahepatic disease. Extrahepatic disease in patients with colorectal hepatic metastases is a major determinant of survival; PHR has a direct association when extrahepatic disease is absent.