AIM: To investigate vascular changes in different stages of glaucoma and to evaluate the role of optical coherence tomography angiography (OCTA) in the early diagnosis of glaucoma. METHODS: Glaucoma patients and healthy controls (n=29 eyes) were investigated in this cross-sectional comparative study. Glaucoma patients were grouped as ocular hypertension (n=44 eyes), preperimetric glaucoma (PPG; n=32 eyes), early glaucoma (EG; n=35 eyes), moderate stage glaucoma (MG; n=36 eyes), and advanced glaucoma (AG; n=35 eyes). Peripapillary and macular vascular densities (VDs) of all participants were compared and correlations of VDs and retinal nerve fiber layer thickness (RNFLT), ganglion cell analysis (GCA), and visual field (VF) tests were evaluated. Area under the receiver operation characteristic curves (AUC) of the peripapillary and macular VD parameters were obtained. RESULTS: VD values ​​decreased with the progression of glaucoma. Most peripapillary and macular VD parameters of PPG and EG groups were lower than healthy controls (P<0.001). There was no significant difference in RNFLT between the PPG and EG groups, but most peripapillary and macular VDs were found to be lower in EG group than in PPG group (P<0.05). In most disease group, VDs were significantly correlated with OCT parameters (P<0.001) and VF index (P<0.05). There were no significant correlations between VF and RNFLT indices in the AG group, but significant correlations were found between VF and VD values (P<0.05). AUC for discriminating between healthy and glaucomatous eyes were highest in whole image peripapillary VD (AUC: 0.865, 0.929, and 0.986, respectively in EG, MG, and AG groups). CONCLUSION: OCTA can be used in the early diagnosis of glaucoma and can be useful in follow-up of the advanced disease. In cases where limitations or suspicions in structural and functional tests are present, OCTA can be used as a supportive diagnostic test, both in EG and AG.