We aimed to investigate the similarities and differences in neuropsychological test performance, demographic features and behavioral patterns of children and adolescents with the attention deficit hyperactivity disorder combined type (ADHD-C), and the severe mood dysregulation (SMD). Study includes 112 children: 67 with ADHD-C, 24 with SMD and 21 healthy controls. These groups were identified by using the schedule for affective disorders, and schizophrenia for the school-age children-present and lifetime version (KSADS-PL) and the K-SADS-PL-SMD Module. Conners' Parent and Teacher Rating Scale-revised long form (CPRS-R:L and CTRS-R:L) and neuropsychological tests were administered to the research groups. ADHD-C group's performances in Wisconsin Card Sorting Test, Trail Making Test, Stroop Test TBAG form and Controlled Oral Word Association Test were significantly poorer than the control group's performances (p < 0.05). Performance of the SMD group was only descriptively intermediate between performances of the ADHD-C and control group. In the "Oppositional", "Hyperactivity", "Social Problems", "Impulsive", "Emotional Lability" and "Conners' Global Index" subscales of CPRS-R:L, the average scores of the SMD group were significantly higher than the ADHD-C and control group's average scores (p < 0.05). ADHD-C group (but not SMD) could be significantly differentiated from healthy controls with the neuropsychological tests used. SMD group could be differentiated from the ADHD-C and healthy control groups with CPRS-R:L; i.e., ADHD-C versus SMD could be differentiated at the behavioral level only.