BACKGROUND The household transmission of hepatitis B virus (HBV) is a major health problem. The prevalence rate of this infection is reported about 11% to 57% among family members of HBsAg carriers. OBJECTIVE This study was conducted to evaluate serological determinants of chronic hepatitis B infection, especially HBsAg positivity, among family members of asymptomatic HBsAg positive carriers in Zahedan (Southeast of Iran). METHODS In a cross-sectional study, data were collected from the total number of 454 HBsAg positive cases and 1817 members of their family by trained interviewers and a validated questionnaire. In addition, blood samples were obtained and titrated to detect serologic markers of hepatitis B. All subjects were recruited following informed consent to the study. RESULTS In total, 454 chronic HBsAg carriers (66% male) with mean age of 36 ± 10 years and 1817 of their family members were included in the study. The prevalence rate of HBsAg and HBcAb positivity among household members were 19.3% (n = 351) and 51% (n = 573), respectively. The frequency of HBV markers was different by age groups. The highest prevalence rate of HBsAg (34.9%) and HBcAb (31.9%) positivity were found in the age group of 21-30 years old. Importantly, the mothers of index cases had the highest prevalence of HBsAg positivity compared to their spouses who had the lowest proportion (53.2% vs. 8.4%, P < 0.001). CONCLUSIONS In family members, HBsAg positivity was four times greater than the general indigenous population. Considering the importance of close contacts for transmission, it was more prevalent in mothers of index cases compared to their spouses, suggesting more efficient mother-to-child than sexually transmission of HBV. It was also more prevalent in those having a history of hepatitis B in their maternal family compared to those with paternal one, probably due to more efficient transmission from infected mothers to children. The lower prevalence of HBsAg positivity in lower age groups could be attributed to vaccination of Iranian infants since 1993 and the screening of HBsAg positive mothers during the last two decades.