The objectiveis to measure the level of fatty acids in erythrocyte membranes and serum of patients with colorectal cancer.Materials and methods.The study group included 100 patients with diagnosed colorectal cancer (57 men and 43 women). The control group included 24 reasonably healthy people (14 men and 10 women) matched for age and sex, without malignant cancers or manifested pathology of the internal organs.Results.Decreased levels of saturated, monounsaturated fatty acids and increased levels of polyunsaturated fatty acids (PUFAs) in erythrocyte membranes and serum (p <0.0001–0.05) were observed. The levels of omega-3 PUFAs in colorectal cancer exceeded the levels in healthy individuals both in erythrocyte membranes and in serum; for omega-6 PUFAs only a trend was observed. At the same time, the ratio of omega-6/omega-3 PUFAs in colorectal cancer was lower than in control (p <0.0001–0.002). The state of erythrocyte membranes more significantly and for more parameters characterized differences between the groups than serum. The most discriminating parameters between patients with colorectal cancer and healthy individuals both in erythrocyte membranes and serum were the levels of C20:2;11,14 (eicosadienoic), C20:3;8,11,14 (dihomo-γ-linolenic), C20:4;5,8,11,14 (eicosatetraenoic, arachidonic), C22:5;7,10,13,16,19 (docosapentaenoic), and C22:6;4,7,10,13,16,19 (docosahexaenoic) PUFAs.