Objective To investigate the changes and clinical significance of plasma copeptin in patients with obstructive sleep apnea hypopnea syndrome(OSAHS)and OSAHS associated hypertension. Methods 56 OSAHS patients who underwent standard polysomnography were enrolled and divided into two groups: 29 patients with OSAHS without hypertension(OSAHS group)and 27 patients with OSAHS associated hypertension(OSAHS+HT group). 30 individuals who had no OSAHS and hypertension patients were included as control cases(N group). The differences in plasma copeptin, polysomnographic characteristics, mean arterial pressure before sleep(MAP_n)and after wakening(MAP_d)were compared between groups. Multiple linear regression analysis was used to explore the factors of copeptin. Results The differences in AHI, ODI, Ts90%, LAT, MSaO₂, and LSaO₂ were statistically significant among the three groups(all P<0.001). The differences in plasma copeptin concentrations among control group, OSAHS group, and OSAHS+HT group were significant. There were significant differences between any two groups(P<0.001). The significant differences in MAP_n and MAP_d were observed among three groups(P<0.001). In the polysomnographic characteristics, the correlation between copeptin and AHI was the strongest(r=0.608, 0.802)in OSAHS group and OSAHS+HT group. Copeptin were correlated positively with MAP_n and MAP_d in OSAHS group and OSAHS+HT group as well(all P<0.05). Multiple linear regression analysis demonstrated that the major factors affecting plasma copeptin were AHI, MAP_n and ODI, successively(all P<0.05). Conclusion The disturbance of the function of arginine vasopressin system is common in patients with OSAHS and OSAHS associated hypertension. Copeptin seems to be involved in the occurrence and development of OSAHS associated hypertension.