In this study, we investigated the virological characteristics, HBV DNA levels and presence of mutations of "a" determinant in the HBsAg S gene in chronic hepatitis B patients with coexisting HBsAg and anti-HBs. The 18 patients who were diagnosed with chronic hepatitis B were both positive for HBsAg and anti-HBs. HBV Among them, 15 patients were DNA positive. The median of HBV DNA levels in serum was $2.18{ imes}10^7$ copies/mL with the HBsAg+/anti-HBsAb+ patients. Also, 4 of 8 HBeAg negative patients had HBV DNA levels higher than $10^4$ copies/mL and the median of HBV DNA levels was $2.03{ imes}10^6$ copies/mL, which were significantly high. These results showed that viral replication still existed in most of the patients of the concomitant HBsAg and anti-HBs, and even in the some HBeAg negative patients. Furthermore, mutation within the "a" determinant of HBV were found in 7 of 15 patients. The most frequent changes were located at positions aa126. In addition, one mutation observed for HBsAg only positive.