Objective:To investigate the clinical significance of serum 25-hydroxyvitamin D [25(OH)D] level in patients with connective tissue disease (CTD) related-pulmonary arterial hypertension (PAH).Methods:CTD patients with PAH (CTD-PAH) and without PAH (CTD-non-PAH) were colle-cted. All data were analyzed.Results:The serum 25(OH)D in the CTD-PAH group was significantly lower than that in the CTD-non-PAH group [(14±8) ng/ml vs (20±8) ng/ml, t=-5.94, P<0.001]. The 25(OH)D deficiency rate in the CTD-PAH group 86.2%(112/130) was significantly higher than that in the CTD-non-PAH group 57.7% (75/130) ( χ2=26.07, P<0.001), while the insufficiency rate was significantly lower [10.0%(13/130) vs 32.3% (42/130), χ2=19.39, P<0.001]. Serum 25(OH)D levels in the systemic lupus erythematosus (SLE), systemic sclerosis (SSc) associated PAH group were lower than those in the SLE [14(8, 17) ng/ml vs 19(15, 23) ng/ml, Z=-3.66, P<0.001], SSc [11(8, 17) ng/ml vs 24(18, 30) ng/ml, Z=-4.97, P<0.001] without PAH group. The levels of serum 25(OH)D in CTD-PAH youthful group, in the middle age group were lower than that in CTD-non-PAH youthful group [(12±8) ng/ml vs (19±8) ng/ml, t=-4.36, P<0.001] and in the middle age group [(14±7) ng/ml vs (21±8) ng/ml, t=-3.75, P<0.001]. Serum levels of 25(OH)D [ OR (95% CI)=1.100 (1.058, 1.144), P<0.001], uric acid [ OR(95% CI)=0.996(0.993, 0.998), P=0.003], immune globulin (Ig)G [ OR(95% CI)=1.123(1.057, 1.194), P<0.001] were associated with PAH in CTD patients. Serum 25(OH)D was positively correlated with calcium ( r=0.24, P=0.007), while negatively correlated between serum 25(OH)D and IgM ( r=-0.34, P<0.001). Conclusion:The occurrence and development of CTD-PAH may be related to the decrease of 25(OH)D level. Serum 25(OH)D level is associated with PAH in CTD patients.