BACKGROUND: Angiotensin-converting enzyme 2 (ACE2), a primary component of the vasoprotective axis in the renin-angiotensin system (RAS), has recently been found to have regulatory actions in hypoxic pulmonary hypertension and monocrotaline-induced pulmonary hypertension. We explored the hypothesis that the level of ACE2 protein contents may be decreased in patients with pulmonary arterial hypertension (PAH) due to congenital heart disease (CHD). OBJECTIVE: We observed the serum ACE2 protein contents in patients with PAH due to CHD (CHD-PAH), and investigated their correlation with mean pulmonary arterial pressure (mPAP). METHODS: One hundred and four patients with CHD and 33 normal control patients (group A) were involved in the research. The patients with CHD were divided into 55 cases of nonpulmonary hypertension (group B), 25 cases of mild to moderate pulmonary hypertension (group C) and 24 cases of severe pulmonary hypertension (group D). The serum level of ACE2 protein contents were detected by enzyme-linked immunosorbent assay (ELISA), and the relationship between these contents and mPAP were analyzed. RESULTS: ACE2 protein contents significantly declined as mPAP increased. The mPAP was negatively correlated with the level of ACE2 protein contents. CONCLUSIONS: These results demonstrated that ACE2 may play an important regulatory role in CHD-PAH.