Objective To investigate the prevalence of anti-endothelial cell antibodies (AECAs) in the sera of connective tissue diseases (CTD) patients with pulmonary arterial hypertension (PAH) and its correlation with clinical manifestations. Methods AECAs in sera of 39 CTD patients with PAH, 22 CTD patients without PAH, and 10 healthy donors as controls were detected with Western blotting. The prevalence of different AECAs in different groups was compared and its correlation with clinical manifestations was also investigated. Results The prevalence of AECAs was 82.1% in CTD patients with PAH, 72.7% in CTD patients without PAH, and 20.0% in healthy donors. Anti-22 kD AECA was only detected in CTD patients with PAH (15.4%). Anti-75 kD AECA was more frequently detected in CTD patients with PAH than in those without PAH (51.3% rs. 22.7%, P