Background: Occult hepatitis B infection (OBI) is a form of hepatitis in which there is an absence of detectable HBsAg, despite the presence of HBV-DNA in the peripheral blood of patients. It seems that non-effective or attenuated immune system responses against HBV lead to the development of OBI. Previous studies showed that the Fas/Fas ligand (FasL) system is an important death signaling pathway that is used by cytotoxic T lymphocytes to eradicate HBV from the liver.Objectives: To investigate polymorphisms in the -670 region of the Fas gene in those with OBI.Patients and Methods: The plasma samples from 3700 blood donors were tested for HBsAg and anti-HBs by ELISA. The HBsAg-/anti-HBc+ samples were selected and screened for HBV-DNA by PCR. Those with HBV-DNA were diagnosed as OBI and PCR-RFLP technique was performed to examine polymorphisms within their Fas gene.Results: 352 (9.5%) of 3700 blood samples were HBsAg-/anti-HBc+. HBV-DNA was detected in 57 (16.1%) of 352 HBsAg-/anti-HBc+ samples. Therefore, 57 HBsAg-/anti-HBc+/HBV-DNA+ patients were diagnosed as OBI. Patient and control groups had no significant differences in terms of the studied polymorphisms. Conclusions: The functional polymorphisms in the promoter region of Fas gene are not associated with OBI. Therefore, it may be concluded that polymorphisms at the -670 position of the Fas gene do not have any critical effects on the immune response against HBV in OBI.