Granulocyte-macrophage-colony stimulating factor (GM-CSF) belongs to the group of glycoproteins called colony-stimulating factors (CSFs). It has been shown that the activity of CSFs is not limited to the hematopoietic cells but can also affect the proliferation of colon carcinoma cell lines. The purpose of this investigation was to compare the serum level of GM-CSF in colorectal cancer patients to a control group, to assess the level of GM-CSF in relation to the level of carcinoembryonic antigen (CEA) and carbohydrate antigen 19-9 (CA 19-9), and to define the sensitivity, the specificity and the predictive values of GM-CSF in colorectal cancer. In this study, the serum level of tumour markers was measured in 30 patients with colorectal cancer and in 20 healthy subjects. GM-CSF was assayed using ELISA system, CEA and CA 19-9 were measured by MEIA. The serum levels of CEA, CA 19-9 and GM-CSF were higher in the patients with colorectal cancer than in the control group. The sensitivities of CEA (63%) and CA 19-9 (56%) were lower than the GM-CSF sensitivity (80%). The specificities of tumour markers were 70% (CEA, GM-CSF) and 75% for CA 19-9. The GM-CSF predictive v values were higher than the CEA and CA 19-9 values. These results suggest that GM-CSF may be useful as tumour marker in colorectal cancer, but further studies are needed.