The presence of hepatitis B virus (HBV) DNA in case of negative HBV surface antigen (HBsAg) in serum is known as "occult hepatitis B". There are many reports indicating that occult HBV infections are more frequently encountered in case of hepatocellular carcinoma, hemodialysis practice and co-infections with hepatitis C virus (HCV). The aim of this study was to investigate the presence of HBV-DNA in HBsAg negative hemodialysis patients and subjects who had never experienced hemodialysis. A total of 226 HBsAg negative sera were included to the study, of which 153 were from hemodialysis patients (97 male, 56 female; mean age: 41.3 +/- 5.8 years), and 73 were from non-hemodialyzed individuals (46 male, 27 female; mean age: 36.5 +/- 6.9 years) who had serological evidence of previous HBV and HCV infections. Of these 73 subjects, 41 were anti-HCV positive, 22 were "anti-HBc IgG positive alone", seven were anti-HBc IgG and anti-HBs positive, and three were anti-HBc IgG and anti-HBc positive, while 40 of 153 (26.1%) hemodialysis patients were anti-HCV positive. HBV and HCV markers were detected by commercial enzyme immunoassays (bioMerieux, France and Murex, UK, respectively), and HBV-DNA testing was performed by a commercial real-time polymerase chain reaction (PCR; 5700 and 7700 Sequence Detection System, Applied Biosystems, UK) assay. Nineteen (12.4%) of HBsAg-negative hemodialysis patients and five (6.8%) of the non-hemodialyzed subjects were found positive for HBV-DNA (viral loads were >= 10(4) copies/ml, and 103104 copies/ml, repectively). The rates of occult HBV infection in the anti-HCV positive hemodialysis patients and anti-HCV positive non-hemodialyzed subjects were detected as 27.5% (11/40) and 2.4% (1/41), respectively. These rates in the other groups were found as follows; 7.1% (8/113) in the anti-HCV negative hemodialysis patients, 9.1% (2/22) in the "anti-HBc positive alone" subjects, and 20% (2/10) in the subjects positive for anti-HBc+anti-HBs or anti-HBe. The results of this study indicated that the prevalence of HBV viremia (12.4%) in hemodialysis patients being more prominent in those of anti-HCV positive patients (27.5%) should not be overlooked. In conclusion, the hemodialysis patients should be screened by sensitive PCR-based methods for occult HBV infections, even if they were negative for H13sAg, in order to prevent or at least to decrease the transmission risk of HBV infection which is still an important health problem in dialysis units.