In addition to clinical examination and various diagnostic procedures, patients with gastrointestinal cancer (GIC) were also monitored by tumor marker (TM) determination. In total, 202 patients with GIC were postoperatively followed-up. According to in vivo diagnostic procedures, there were 133 (66%) patients without metastases, 63 (31%) patients with distant metastases, 48 (76%) of them with liver metastases, and six (3%) patients with local recurrences. During the 1990-1995 period, they were followed-up by serum TM concentration measurements on 1-8 occasions. At the time of initial diagnosis, TM were determined in a varying percent of the patients: CA 19-9 (100%), TPA (54%), CA 72-4 (49%), IAP (41%), CEA (41%) and AFP (25%). In the group of patients without metastases, the percentage of normal TM values ranged between 73%-100%, and TM sensitivity between 0%-86%. No ideal TM has as yet been discovered either for any malignant disease or for patients with GIC. Therefore, in the follow-up of patients with GIC we suggest the concentrations of at least two instead of only one TM (CA 19-9 and TPA or CEA) to be determined. It is also difficult to choose the best TM among numerous TM kits available on the market and to keep using it during a longitudinal follow-up study.