This study was designed to explore the incidence rate of occult HBV infection in patients with anti-HBc positive alone and analyze the possible reasons of occult infection.Sera of 183 patients carrying anti-HBc alone(A < or = 0.1) were collected and real-time PCR was used to select samples with HBV DNA positive. HBV pre-S/S amplification products were obtained by PCR, and clonal sequencing were then used for these samples with HBV DNA positive.DNA quantitative results of three samples were greater than 10(3) copies/ml in 183 samples, with a fraction of 1.6%. Pre-S/S sequencing results of two samples from these three samples were obtained. Point mutations within "a" determinant with Q129R/P mutations and co-existence of the mutant type and wild type were found in the two samples.Occult HBV infection existed in samples with anti-HBc alone. Factors contributing to the loss of HBsAg detection by immunoassays include S gene mutations and low levels of circulating antigen which are below the assay limit of detection. Occult HBV infection not only can lead to a false clinical diagnosis, but also can result in hematological pollution due to such occult infection of blood donors.