Objective: In our study, we aimed to evaluate the plasma homocysteine levels and oxidative stress parameters in patients with adult ADHD (A-ADHD). Method: Blood samples were taken from 32 A-ADHD patients and 32 controls enrolled in the study and plasma homocysteine, serum folate, vitamin B12, total antioxidant status (TAS), total oxidant status (TOS), and oxidative stress index (OSI) were determined. Results were compared between the groups. Results: Patients’ plasma homocysteine level was significantly lower and serum folate level was significantly higher than the controls. The means of plasma homocysteine and folate levels were statistically significantly different between the groups (p < .05). However, a statistically significant difference was not detected between groups for mean serum vitamin B12, TAS, TOS, and OSI levels (p > .05). Conclusion: Our study suggests that oxidative balance is not impaired in A-ADHD, and contrary to other psychiatric disorders, homocysteine levels were found to be lower.