Accumulated studies have shown that ω-3 polyunsaturated fatty acids such as eicosapentaenoic acid (EPA) have protective roles against inflammatory responses such as hyperlipidemia, diabetes mellitus (DM) and cardiovascular diseases. Here we examined the effects of administering EPA to hyperlipidemic patients and other patients undergoing cardiac surgery to determine whether this treatment would increase plasma EPA levels and to clarify the association between EPA treatment and adiponectin production in hyperlipidemic patients. We also assessed the effect of preoperative EPA administration on postoperative adverse events such as postoperative atrial fibrillation (POAF) and postoperative infection in the cardiac surgery patients. The EPA administration significantly increased the serum EPA concentrations in both patient populations (p＜0.001). In the hyperlipidemic patients, the EPA administration significantly increased plasma adiponectin levels (p＜0.05), accompanied by a decrease in insulin resistance designated by the HOMA-IR (homeostasis model assessment of insulin resistance) score (p＜0.05) and Hs-CRP (high sensitivity C-reactive protein) value (p＜0.05). In the cardiac surgery patients, no significant effect of EPA on cardiac adverse events such as POAF was observed. However, our results clearly demonstrated that both the neutrophil-to-lymphocyte ratio and the 2nd-line antibiotic requirement in the EPA group were significantly decreased compared to the untreated control group (p＜0.05). We suggest that EPA administration may exert anti-inflammatory effects in patients with hyperlipidemia and in those undergoing cardiac surgery, possibly through an increase in plasma adiponectin levels.