The rates of growth of 29 hepatic metastases from 15 patients with primary colorectal carcinoma were studied using serial computed tomography (CT). Eleven metastases were found by the surgeon at laparotomy (overt metastases); the remaining eighteen were not evident to the surgeon at laparotomy, but were detected by CT scan during the immediate postoperative period (occult metastases). An estimate of tumour volume doubling time was obtained from a semi-logarithmic plot of tumour cell number against time. The mean doubling time for the overt metastases was 155 +/- 34 days (+/- s.e.m.) compared with 86 +/- 12 days (P less than 0.05) for the occult metastases. The age of the metastases at the time of surgery was estimated by extrapolation of the observed growth curve assuming Gompertzian kinetics. The mean age of the overt metastases was 3.7 +/- 0.9 years (+/- s.e.m.) The corresponding age of the occult metastases was 2.3 +/- 0.4 years.