Neopterin (NP) is a pteridine derivative that is secreted as a response to gamma interferon stimulation. The purpose in this study was to investigate the relationship between the NP levels and the disease and determining the efficacy of an interferon (IFN) therapy in patients with chronic hepatitis B (CHB) and C (CHC). The study was conducted on 49 cases with CHB, 30 cases with CHC and 72 healthy individuals. Serum samples were taken from the patients receiving treatment at the beginning and at the end of the treatment and only once from the healthy individuals in the control group. The NP levels were found significantly higher in the patients with CHB and CHC than those in the control group. When the pre and post-treatment serum NP levels of the patients who received an interferon therapy were compared, the post-treatment NP levels of the patients who responded to the treatment were significantly higher. When a comparison was made before and after treatment, a decrease was seen in the NP levels in most of the infections due to decreased activation of the immune system. However, when the disease was treated with an IFN therapy, which is a treatment stimulating the immune system, the post-treatment NP level remained high.