BACKGROUND: Although the development of antibodies(anti-HBs) against hepatitis B surface antigen(HBsAg) generally leads to the clearance of the infecting hepatitis B virus(HBV), anti-HBs reactivity has been reported in patients with chronic hepatitis. Some reports suggested that concurrent presence of HBsAg and anti-HBs was a common serologic pattern in HBV carriers, however, the others insisted concurrence was associated with evidence of viral replication and features of active inflammation. This study was to investigate the frequency and significance of concurrent presence of HBsAg and anti-HBs among Korean HBsAg-positive patients. METHODS: HBV serological markers were analyzed by Abbott Axym. Anti-HBs titer was assessed quantitatively using 6 standard calibrators(0, 10, 50, 100, 500 & 1,000 mIU/mL), and reported as positive when more than 10 mIU/mL. RESULTS: Of 959 consecutive HBsAg-positive patients who had simultaneous anti-HBs determinations, HBsAg and anti-HBs were found concurrently in 31(3.2%). Anti-HBs was quantitated 10-100 mIU/mL in 25(80.6%) and more than 100 mIU/mL in 6(19.4%) among 31 HBsAg-positive patients with concurrent presence of anti-HBs. HBeAg positive rate of concurrent HBsAg/anti-HBs-positive patients(15/22 or 68.2%) was higher than that of HBsAg-positive patients without anti-HBs(227/562 or 40.4%). Of 31 HBsAg-positive patients with concurrent anti-HBs, 15 were patients with chronic hepatits, 5 with primary hepatocellular carcinoma and 2 with liver cirrhosis. CONCLUSIONS: Concurrent HBsAg/anti-HBs-positive patients were found in about 3% of Korean HBsAg-positive patients and their anti-HBs titers were less than 100 mIU/mL in 80% of concurrent HBsAg/anti-HBs-positive patients. In patients with chronic hepatits B who possessed HBsAg and anti-HBs above 100 mIU/mL, infections of HBV variants with mutations in the envelope gene might be suspected.