The study included children between the ages of 6 and 11 years (mean [SD] age: ADHD: 8.19 [1.27] years; ADHD + ODD: 8.06 [1.06] years; control group: 8.47 [1.31] years) who were diagnosed as ADHD and ADHD comorbid with oppositional defiant disorder (ADHD + ODD) according to the DSM-IV diagnostic criteria. The ADHD, ADHD + ODD, and the control group included 21, 26, and 27 male subjects, respectively. We used the CBCL (Child Behavior Check List) to obtain standardized parental reports of children's problem behaviors. A 10-cc blood sample was taken from every child in the morning between 8:15 A.M. and 9:00 A.M., and plasma free T3, T4, and TSH levels were investigated with Immulite-2000 Apparat (Diagnostic Products Corporation, Los Angeles, CA) after the samples were centrifuged for 10 minutes at 3,000 rpm. There were no significant differences between the ADHD, ADHD + ODD, and the control group in terms of average age, IQ, sociodemographic data, and comorbid LD (p>0.05).