Objective: Increasing evidence suggests that certain inflammatory factors, especially cytokines, can lead to neurochemical and behavioural changes via their role in the brain signalling system. Studies investigating cytokine abnormalities in obsessive-compulsive disorder (OCD) have yielded conflicting results thus far. The aim of this study was to investigate the serum IL-1, IL-4, IL-6, IL-10, TNF-alpha, and IFN-gamma levels in drug-free, comorbidity-free patients with OCD. Methods: Forty comorbidity-free, drug-free OCD patients and 40 age-and sex-matched healthy volunteers were included in the study. Clinical severity was assessed using the Yale-Brown Obsessive-Compulsive Evaluation Scale (YBOCES). Results: IL-1 levels were significantly higher and IL-2, IL-6, and TNF-alpha levels were significantly lower in patients than in controls. No differences were observed in IL-4, IL-10, and IFN-gamma levels. However, a negative relationship was found between IL-1 levels and compulsive symptoms (YBOCES) in OCD patients. Conclusions: Our findings suggest the possible role of cytokines in the pathogenesis of OCD. It is important to continue investigating the role of cytokines and other immunological markers in both the etiology and treatment of OCD.