Attention deficit hyperactivity disorder (ADHD) is one of the most prevalent psychiatric and developmental disorders among children and adolescents. Besides clinical impairments, these children have challenges with school performance and independent socioeconomic factors. The aim of this study was to assess the possible effects of vitamin D on inflammatory cytokines (tumour necrosis factor-alpha (TNF-α) and interleukin-6 (IL-6)) levels in children with ADHD. This study is a randomised, double-blind, placebo-controlled clinical trial which was conducted on 86 children aged 6–12. Patients were randomly assigned to two groups to receive vitamin D3 or a placebo. Results show children with ADHD taking vitamin D supplementation for 3 months demonstrated a significant increase in serum levels of Vitamin D3. However, serum levels of IL-6 and TNF-α were not significantly influenced by vitamin D administration. Authors conclude that their findings failed to find a favourable effect of 3 months of supplementation with vitamin D on inflammatory cytokines. Thus, they highly recommend conducting further studies with different doses of vitamin D and various designs in addition to differences in the characteristics of participants.