Bacground/Aim. Using omega-3 polyunsaturated fatty acids (PUFAs) in coronary artery bypass graft surgery (CABG) could provide protection against ischemicreperfusion damage, prevention of postoperative arrhythmia and attenuation of inflammatory response. However, omega-3 PUFAs inhibit cyclooxygenase (and thus decrease the synthesis of thromboxane A2 from arachidonic acid in platelets), which leads to decreased platelet aggregation. In cardiac surgery it is necessary to achieve a balance between inhibition and full platelets function. It is as well as important to closely follow hematological parameters, impaired by CABG itself. Therefore, the aim of the study was to establish the effects of pretreatment with omega- PUFAs on hematological parameters and plateletes aggregation in patients with elective CABG. Methods. This prospective, randomized, placebo-controlled, single-center trial was performed on parallel groups. The patients (n = 40) undergoing elective CABG were randomized receiving preoperative intravenous omega-3 PUFAs (Omegaven® 10%) infusion (the PUFAs group) or the same volume of 0.9% saline solution infusion (the control group). Infusion was given a day before surgery and repeated four hours before starting extracorporeal circulation (CPB) via the peripheral vein at single doses of 100 mL (25 mL/h). Platelet function analysis was performed using multiple electrode aggregometry (MEA, multiplate-analyzer) before starting CPB and 2 h postoperatively for the patients of both groups. Results. There were no clinically relevant differences in baseline characteristics between the groups. Hematological parameters were not significantly different between the groups pre-, intra- and postoperatively. During the first 24 h after surgery, the loss of blood was similar in the PUFAs and the control group (680 ± 274 mL and 608 ± 210 mL, respectively; p = 0.356). Postoperatively, platelet aggregation was not significantly different between the PUFAs and the control group in adenosine diphosphate (ADP) test (39 ± 11 and 42 ± 15, respectively; p = 0.701), arachidonic acid (ASPI) test (64 ± 24 and 70 ± 27, respectively; p = 0.525) and trombin receptor-activating peptide (TRAP) test (68 ± 25 and 75 ± 26, respectively; p = 0.396), while their aggregation in collagen (COL) test was statistically significantly lower in the PUFAs related to the control group (32 ± 15 and 47 ± 20, respectively; p = 0.009). Conclusion. Acute pretreatment with omega-3 PUFAs insignificantly affected the activity of platelets and did not influence postoperative blood loss.