The aim of this study was to investigate the clinical value of CA 242 and CEA in the follow-up of patients with colorectal cancer. Serial serum samples were available for analysis in 67 patients with subsequent recurrence, of which 36 patients had been treated for colonic and 31 patients for rectal cancer. Liver metastases were found in 32 patients, local recurrences in 18 patients, lung metastases in 11 patients and other distant metastases in 6 patients. The same serum samples were used in quantitating the serum levels of both CA 242 and CEA. At the time of clinical recurrence an elevated CA 242 level was found in 41 patients and an elevated CEA level in 49 patients. Thirty-six patients (54%) showed an elevation of both CA 242 and CEA, five patients (7%) had increased CA 242 alone and 13 patients (19%) increased CEA alone. Altogether, 54 patients (81%) showed an elevation of either or both markers at the time of clinical recurrence. Initially CA 242 alone began to rise in 14 patients (21%) and CEA alone in 16 patients (24%). The lead time was calculated from 28 patients that had four or more serum samples available during follow-up. CA 242 increased in median 5,7 months and CEA in median 3,4 months before clinical recurrence (p=0.34). CA 242 was more sensitive for lung metastases (64%) than CEA (45%), whereas CEA was superior to CA 242 in liver metastases (88% versus 72%, respectively) and in local recurrences (56% versus 39%, respectively). Both CA 242 and CEA seem to be useful in early diagnosis of a recurrence in the follow-up of patients with colorectal cancer.