AIM:To measure changes of total oxidant status (TOS) and total antioxidant capacity (TAC) of aqueous humor (AH) in diabetic retinopathy (DR) patients, and to determine if there were any differences in TOS and TAC of AH in diabetic patients without retinopathy compared with non-diabetic patients. METHODS:One hundred and three eyes of 103 patients who were enrolled for cataract surgery were included in this study. Patients were grouped according to presence of diabetes and stage of DR. Prior to cataract surgery, 0.1mL to 0.2mL of AH was aspirated and analyzed for TAC and TOS level using a colorimetric method. RESULTS:TOS levels were highest among proliferative diabetic retinopathy (PDR) patients and lowest in patients with only cataracts. Results were statistically significant between all groups (P<0.05). Whereas result between diabetic without retinopathy patients and non-proliferative diabetic retinopathy (NPDR) patients was not statistically significant (P=0.757). TAC levels were highest in patients with only cataract and lowest among PDR patients and results were statistically significant between all groups (P<0.05). CONCLUSION:Aqueous humor TAC levels are low in diabetic patients and reduced further in DR patients, TOS levels are increased in diabetic patients and this is exacerbated in DR patients.