Tumour progression in 340 patients with resected gastrointestinal primary tumours was monitored using the gradual increase in carcinoembryonic antigen (CEA) in serum. The commencement of the rise in CEA generally preceded clinical detection of the cancer by several months. The degree to which the rise in CEA correlated with the recurrence of cancer was investigated. There was a marked difference in the distribution of the rises in CEA between local tumour growth and distant metastases. CEA increases of more than 1 microgram CEA/l serum in 10 days occurred exclusively in patients with distant metastases. There was a further marked difference in the distribution of the CEA increase between the group with liver metastases and the groups with peritoneal carcinomatosis or other metastases. The site of the primary tumour had no influence on the CEA increase during formation of metastases.