There is preliminary evidence that the immune system's cytokines may have impact on ADHD in children. Nevertheless, studies exploring the possible role of pro-inflammatory cytokines in adults with ADHD are lacking. This study aimed to assess differences in serum IL-6 and TNF-alpha between patients and controls and their possible relationship to resting cortisol. 108 adults with ADHD (DSM-IV), 44 inattentive and 64 combined, age ranging between 18 and 55 years, and 27 healthy controls were included. Major psychiatric disorders and organic comorbidities were excluded. Serum samples for IL-6 and TNF-alpha and salivary samples to assess cortisol awakening response were collected on the same day. Analysis of variance was applied to study differences in IL-6 and TNF-alpha between groups. Pearson correlations were used to study associations between IL-6, TNF-alpha, and CAR. There were no significant differences in serum IL-6 or TNF-alpha levels between patients and controls or between combined and inattentive patients. Negative associations between IL-6 (r=-0386, p=0.020), TNF-alpha Cr=-0.372, p=0.023) and cortisol awakening response were found in the inattentive subtype, whereas no association was seen in the combined subtype. A negative correlation between 1L-6 and cortisol was also present in the control group (r=-0.44, 0.030). The peripheral pro-inflammatory markers, IL-6 and TNF-alpha, do not appear to be primarily involved in ADHD in adults, although the role of other inflammatory markers cannot be ruled out. The differences regarding the association between IL-6 and TNF-alpha and morning cortisol response suggest possible underlying neurobiological differences between the inattentive or combined patients that merit further studies. (C) 2017 Elsevier Ltd. All rights reserved.