This study aims to explore the correlation between T-lymphocyte subsets and hemodynamics in patients with congenital heart disease-associated pulmonary arterial hypertension (CHD-PAH). Thirty patients with CHD-PAH from Cardiovascular Hospital Affiliated to Shanxi Medical University were included and divided into cardiac function class I to II (low-risk group) and cardiac function class III to IV (high-risk group) according to the World Health Organization functional class. T-lymphocyte subsets levels and hemodynamic parameters related to right heart function were compared between the 2 groups, and correlation analysis of T-lymphocyte subsets levels and hemodynamic parameters related to right heart function was also performed. Color Doppler ultrasonography was used to measure the hemodynamic parameters of the patients, and flow cytometer was used for the detection of T-lymphocyte subsets. The percentage of regulatory T-cells in the high-risk group (3.62 ± 0.72) was lower than that in the low-risk group (4.10 ± 0.48) (P = .039). Compared with the low-risk group, the Th17/Treg and Th2/Treg levels in the high-risk group were significantly increased, and the difference was statistically significant (P <
.05). Right ventricular anteroposterior is negatively correlated with regulatory T cell percentage count with a correlation coefficient of -0.373 (P = .042), and positively correlated with Th2/Treg with a correlation coefficient of 0.392 (P = .032). Tricuspid annular plane systolic excursion and tricuspid annular velocity of motion are positively correlated with the percentage count of T-lymphocytes, with correlation coefficients of 0.397 (P = .03) and 0.413 (P = .023), respectively. Pulmonary artery systolic pressure demonstrated a significant positive association with absolute count of T-lymphocytes (R = 0.387, P = .034), helper T cells (R = 0.426, P = .019), suppressor T cells (R = 0.466, P = .009), Th2 cells (R = 0.453, P = .012), Th17 (IL-17) cells (R = 0.408, P = .025). Tricuspid regurgitation velocity is positively correlated with absolute counts of Ts cells (R = 0.426, P = .019) and Th2 (R = 0.361, P = .05) cells. The inferior vena cava collapsibility index is positively correlated with the absolute count of Th1 cells (R = 0.388, P = .034). Our study confirmed that changes in T-lymphocyte subsets were associated with hemodynamic changes in patients, suggesting that T-lymphocyte subsets may be involved in the development of CHD-PAH, and that immunomodulatory therapy may become a new direction for the treatment of CHD-PAH in the future.