Aim: Oxidative stress is involved in both Periodontal Disease (PD) and Diabetes Mellitus (DM). The present study aimed to evaluate the oxidative balance in diabetic patients diagnosed with PD before and after non-surgical periodontal treatment. Materials and methods:Sixty patients were divided into three groups all receiving non-surgical periodontal treatment plus either chlorhexidine, ozone-therapy or antioxidant mouth-rinse. Probing depth (PPD), Percentage Plaque Index (%PI) and Percentage Bleeding on Probing (%BoP) were recorded. Free-radicals (dROMs), plasmatic antioxidants (PAT), salivary antioxidants (SAT) and glycated hemoglobin ( HbA 1 c) were measured. Results: Mean PPD at baseline was 3.14 mm. Mean PPD three months after was 2.05 mm. The overall %PI at baseline was 55% and %BOP 76%. Three months after treatment %PI was 34% and %BoP was 64%. The longitudinal analysis did not show differences between groups. Mean dROMs at baseline was 353 U. Carr (oxidative stress) and decreased after three months reaching 295 U. Carr (normal). SAT was 2083 U. Carr at baseline (inflammation) and decreased to 1337 U. Carr (ideal). The longitudinal analysis did not show differences between groups. Mean HbA 1 c at baseline was 6.92% and decreased significantly to 6.63% three months after treatment. Conclusion:Based on the results of the present study, oxidative stress should be further investigated as a potential modulator of the clinical course of both DM and PD.