Research on different subtypes of attention deficit hyperactivity disorder (ADHD) yielded inconsistent results regarding the nature of cognitive deficits. Whereas some studies report significant differences between subtypes, others fail to report these differences. In fact, the majority of studies in the field of ADHD does not differentiate between subtypes at all. The present study adopted the cognitive task of negating valence to compare the DSM-defined ADHD combined subtype (ADHD-C; n = 25), the inattentive subtype (ADHD-I; n = 25) and the control group (n = 30). As a main result, children with ADHD-C showed significant impairments compared to children with ADHD-I and the control group on conscious responses-driven by executive function, as well as on unconscious associative tasks. Medical treatment with stimulants positively influenced cognitive performance, although to a different extent for subgroups. The results are discussed in the context of current theories of ADHD and imply indications for further research in this field.