Analyzing the local pericardial fluid (PF) surrounding the heart can provide insights into the function of myocardial and pericardial epithelial cells. Kynurenine (KYN) metabolites have been associated with endothelial dysfunction and the progression of atherosclerosis. This study investigated the relationship between KYN pathway metabolites in serum and PF and the severity of coronary atherosclerosis, as assessed by the Syntax Score (SS). The study involved 101 patients (mean age: 60 years). Of the participants, 17% were female. KYN and its metabolite levels were measured using API SCIEX 3200 LC-MS/MS methods. Statistical analysis was performed using IBM SPSS 25.0. The mean serum levels of tryptophan (TRP), 3-hydroxykynurenine (3-HK), and 3-hydroxyanthranilic acid were significantly higher (p < 0.01) when compared to the mean PF levels. In the SS-low group, PF IDO activity was significantly higher than serum IDO activity (p = 0.040). In the SS-high group, PF IDO activity was significantly higher than serum IDO activity (p = 0.002). The mean 3-HK measurements showed significant differences between the SS groups (p < 0.05). In this study, higher IDO enzyme activity in the PF compared to systemic circulation suggests the presence of inflammatory activity in the pericardial endothelial cells. This research aimed to uncover the role of amino acid metabolism, which is poorly understood in atherosclerosis. Specifically, 3-HK, a metabolite from the KYN pathway, may contribute to the development of atherosclerosis. These findings indicate that localized kynurenine pathway activation in the PF may contribute to coronary atherosclerosis, highlighting its potential as a biomarker or therapeutic target.