Objective:To investigate the associations between plasma trimethylamine-N-oxide (TMAO) level and premature coronary heart disease (PCHD).Methods:From July 2018 to July 2020, total of 166 patients with suspected coronary heart disease were enrolled from the Heart Center of Shenzhen Bao′an Hospital affiliated to Southern Medical University. According to the coronary imaging results and age of onset, they were divided into young control group ( n=30), PCHD group ( n=49), middle-aged and elderly control group ( n=30) and the middle-aged and elderly coronary heart disease group ( n=57). Plasma TMAO concentration in each group was determined by stable isotope liquid chromatography/mass spectrometry, and the correlation of plasma TMAO level with PCHD and SYNTAX score was analyzed. Results:The plasma TMAO level in PCHD group was significantly higher than that in young control group [(7.54±2.10) μmol/L vs. (4.60±1.89) μmol/L; t=6.73, P?0.001] and middle-aged and elderly coronary heart disease group [(3.90±1.75) μmol/L; t=2.45, P=0.015]. The plasma TMAO level was positively correlated with SYNTAX score in PCHD group ( r=0.66, P?0.001) and in middle-aged and elderly coronary heart disease group ( r=0.27, P=0.042). Multivariate logistic regression analysis showed that plasma TMAO level was an independent risk factor for PCHD ( OR=2.30, P?0.001). Receiver operating characteristic (ROC) curve analysis showed that when the cutoff level of plasma TMAO was 6.08 μmol/L, the sensitivity and specificity for diagnosis of PCHD were 73.5% and 76.7%, respectively. Conclusion:The plasma TMAO level is significantly correlated with PCHD and had certain predictive value for PCHD.