Aim of work is to study the features of the fatty acid (FA) composition of the membranes of erythrocytes (Er) and blood serum (BS) in patients with Crohn’s disease (CD) for possible use for diff erential diagnosis. Materials and methods . Using a gas chromatography-mass spectrometric system (GC/MS) based on three Agilent 7000B quadrupoles (USA), the composition of erythrocytes (Er) and blood serum (SC) of 60 people was studied: 19 — with Crohn’s disease (CD, 34±6,2 years), 21 — with unclassifi ed colitis (UCC, 35,2±4,7 years) and 20 healthy individuals (32,9±5,7 years). Results . It was found that the levels of most saturated, monounsaturated omega-9, omega-6 PUFAs were signifi cantly higher, and the content of the predominant part of omega-3, a number of omega-6 (linoleic, eicosadienoic, arachidonic) was lower than in healthy individuals. The relative content of FA in patients with unclassifi ed colitis turned out to be intermediate between those in patients with CD and healthy (p <0,0001–0,02). The obtained diff erentiating values of FA levels and their indices in the erythrocyte membranes made it possible to identify patients with suspected Crohn’s disease from those who were diagnosed with unclassifi ed colitis, which indicates the high potential of this approach for diff erential diagnosis. Conclusion . The created diagnostic panels (FA of blood serum and erythrocyte membranes) made it possible to diff erentiate patients with CD from healthy individuals with fairly high levels of sensitivity and specifi city. The revealed features of the levels of FA membranes of Er and BS, as well as their ratio in patients with CD, are promising for the purposes of diff erential diagnosis, including in cases of unclassifi ed colitis.