OBJECTIVE Neopterin is a marker of cell-mediated immunity. It also has a fundamental role in host-defense reactions, including interactions with reactive oxygen intermediates and the promotion of local and systemic oxidative stress. The present study aimed to assess the importance of serum neopterin levels in patients with non-alcoholic steatohepatitis (NASH). METHODS Thirty-nine patients with NASH diagnosed by liver biopsy and 32 healthy adults (controls) were enrolled in the study. Serum neopterin levels were measured with an enzyme-linked immunosorbent assay in addition to other biochemical parameters, including liver enzymes. Histopathological examinations were graded as suggested by both the necroinflammatory activity grading system and the NASH scoring system. RESULTS The mean serum neopterin levels were higher in patients with NASH compared to the controls (24.1 +/- 16.4 vs 16.2 +/- 9.5, P = 0.019). The histological examination of liver biopsies revealed that 34 of the patients with NASH had grade 1 steatohepatitis and only five patients had grade 2 steatohepatitis. A higher serum mean neopterin level was detected in grade 2 patients compared to grade 1 (40.6 +/- 5.6 vs 21.7 +/- 16.1, P = 0.014). A gradual increase was also observed in serum neopterin levels with the increase of the NASH score. CONCLUSIONS The serum neopterin levels were significantly higher in patients with NASH compared to the controls, and levels showed an association with the severity of liver damage.