BACKGROUND: After an infection with HBV, HBsAg is the first virologic marker detectable in the serum. If anti-HBs against 'a'determinant of HBsAg appears, HBsAg will disappear and the patients will recover from the HBV infection in most cases. However, we encounter not infrequently concomitant cases of HBsAg and anti-HBs. In this study we evaluated HBV DNA levels in concomitant cases to aid in the interpretation of these serologic results. METHODS: This study included 36 cases with positivity for both HBsAg and anti-HBs in an electrochemiluminescent immunoassay as well as a radioimmunoassay. They were tested for HBeAg, anti-HBe, and HBV DNA levels. RESULTS: Chronic viral hepatitis was the most frequent diagnosis (15/36 : 41.7%) and AST and ALT levels were normal in 17 (47.2%) and 20 (55.6%) cases, respectively, among total 36 concomitant cases. HBeAg was positive in 24 and anti-HBe in 17 cases. HBV DNA was positive in 33 cases (91.7%). including all 24 HBeAg positive cases and 9 (75%) of 12 HBeAg negative cases; 6 (50%) of 12 HBeAg negative cases had HBV DNA levels higher than 105 copy/mL. CONCLUSIONS: This study showed that viral replication still exists in most cases of concomitant HBsAg and anti-HBs, and even in some HBeAg negative cases. So in the concomitant cases, HBV DNA quantitation may aid in the interpretation of clinical significance of these cases.