AIM: To investigate the expression and correlation of C1q/tumor necrosis factor related protein 9(CTRP9)levels in the serum of patients with different stages of diabetic retinopathy(DR)and diabetic macular edema(DME).METHODS: A total of 135 patients with type 2 diabetes who were admitted to Gansu Provincial Hospital from April 2021 to April 2022 were selected as the experimental group. According to the results of non-mydriatic fundus photography, they were divided into non-DR(NDR)group(n=45), non-proliferative DR(NPDR)group(n=45), proliferative DR(PDR)group(n=45); according to the results of optical coherence tomography, DR patients were divided into DME group(n=51), non-DME group(n=39). In addition, other 45 healthy subjects who matched the age and sex of the experimental group were selected as normal control group. The clinical data and biochemical index test results of subjects in each group were recorded and compared, the correlation between serum CTRP9 level and other biochemical indexes was analyzed, and the risk factors affecting the occurrence of DR and DME were explored.RESULTS: There were significant differences in serum CTRP9 levels among subjects in normal control group, NDR group, NPDR group and PDR group(P<0.001), and normal control group > NDR group > NPDR group > PDR group. There was significant difference in serum CTRP9 level between DME group and non-DME group(P<0.001), and non-DME group > DME group. Spearman rank correlation analysis showed that the level of serum CTRP9 in DR patients was negatively correlated with the course of diabetes(rs=-0.251, P<0.05), the level of serum CTRP9 in DME patients was negatively correlated with fasting blood glucose(FBG)(rs=-0.370, P<0.05)and glycosylated hemoglobin(HbA1c)(rs=-0.421, P<0.05). Logistic multivariate regression analysis showed that the course of diabetes(OR=1.194, 95%CI: 1.068～1.335,P=0.002)and the level of serum CTRP9(OR=0.936, 95%CI: 0.907～0.966,P<0.001)were risk factors for DR. The level of serum CTRP9 was a risk factor affecting the occurrence of DME(OR=0.838, 95%CI: 0.778～0.903, P<0.001).CONCLUSION: The reduction of CTRP9 level is a risk factor for the occurrence of DR and DME, which may be of great significance to the risk assessment of both DR and DME.