AIM: The purpose of our study was to investigate the relation between serum Toxoplasma gondii IgG antibodies in children and attention deficit hyperactivity disorder (ADHD) and its severity. METHOD: 214 subjects, consisting of 107 children aged 6-18 and diagnosed with ADHD and 107 children with no ADHD or psychiatric pathology were included. Subjects underwent a detailed psychiatric examination based on DSM-V-TR diagnostic criteria, using a data form, the Kiddie Schedule for Affective Disorders and Schizophrenia Present and Lifetime Version (K-SADS-PL) for School-Aged Children (6-18), the DuPaul ADHD Rating Scale, Parent Rating Scale, the Conners Teacher Rating Scale (CTRS), and the Conners Parent Rating Scale (CPRS). Blood anti-Toxoplasma IgG antibody levels were investigated. The data obtained were then subjected to statistical analysis. RESULTS: T. gondii IgG antibodies were positive in 8 (7.47%) of the case group and positive in 3 (2.8%) of the control group. No statistically significant difference was determined between the case and control groups in terms of T. gondii IgG positivity (p = .215). Higher levels of severe ADHD were determined in Toxoplasma IgG positive patients in the ADHD group compared to Toxoplasma IgG negative subjects, the difference being statistically significant (p = .005). CONCLUSION: No significant differences were determined between the case and control groups in terms of T. gondii IgG positivity and ADHD. However, correlation was determined between ADHD severity and T. gondii IgG positivity.