OBJECTIVES: Fatty acids play a role in development and progression of colon cancer. The aim of this study was to assess the relation between tissue fatty acids (saturated fatty acids, unsaturated fatty acids, the ratio of C18 to C18:1 - index of fatty acids saturation, SI), colorectal tumor localization and disease progression. METHODS AND RESULTS: Total of 49 patients (14 with proximal colon, 13 with distal colon and 22 with rectal tumor localization) were studied. One year after surgery 24 patients had the disease progression. Tissue levels of saturated fatty acids and unsaturated fatty acids were measured before surgery by gas-chromatography. These fatty acids were determined in cancerous tissue (CA) and non-cancerous tissue (NCA). The most significant differences in the mean values of fatty acids of phospholipids between CA and NCA in patients with proximal tumor localization were noted. The mean value of C18 was significantly lower while C18:1 was significantly higher in CA as compared to NCA in patients without disease progression (p<0.02; p<0.03; respectively). SI was significantly lower in CA as compared to NCA only in patients without disease progression (p<0.02). CONCLUSION: Fatty acids of tissue phospholipids' fraction, as well as SI, strongly depend on tumor localization and might be useful as potential markers of the disease progression in colorectal cancer patients.