Background: Approximately 80% of obsessive–compulsive disorder (OCD) cases begin in childhood. Various genetic, psychological, sociological factors and biological mechanisms are involved in the etiology of OCD. To the best of our knowledge the relationship between inflammation and OCD is unclear. Chronic inflammation was shown to increase neopterin and decrease tetrahydrobiopterin (BH4) levels by activating the neopterin–BH4 pathway. In addition, studies have shown that it can be an important biomarker in psychiatric disorders. Objective: This study compared serum TGF-1β, TNF-α, IL-1β, IL-2, IL-6, IL-10, IL-17, neopterin, BH4, and nitric oxide (NO) levels between child and adolescent patients diagnosed with OCD and a healthy control group. Methods: The study included 29 patients diagnosed with OCD (comorbidity free, drug free) and 28 healthy children as an aged and sex matched control group. For the measurement of neurobiological markers, venous blood samples were collected, and analyzed by using enzyme-linked immunosorbent assay (ELISA). Results: All cytokine levels were found to be low, but this decrease was statistically significant only for TGF-1β. The neopterin and NO levels were significantly higher and BH4 significantly lower in children with OCD compared to the healthy control group. Conclusion: The results of our study show that the levels of TGF-1β and NO and the activation of the neopterin–BH4 pathway may be implicated in the pathophysiology of OCD. Additionally, anti-oxidant and BH4 adjuvant therapies should be investigated as treatment options for OCD.