Objective: To define the risk of hepatitis B virus (HBV) transmission through oocytes and embryos from chronic HBV carriers. Design: Laboratory-based study. Setting: Research laboratory in a university hospital. Patient(s): Thirty-one couples with hepatitis B surface antigen (HBsAg)-negative women and HBsAg-positive men, 41 couples with HBsAg-positive women and HBsAg-negative men, and 39 seronegative couples. Intervention(s): None. Main Outcome Measure(s): Hepatitis B virus DNA and RNA analyses in oocytes and embryos, and the location of virus particles containing HBsAgs. Result(s): Hepatitis B virus DNA was detected in 3 of 18 male HBsAg-positive/female HBsAg-negative couples (and in 13 of 84 embryos) and 3 of 14 male HBsAg-negative/female HBsAg-positive couples (and in 15 of 71 oocytes and embryos). Hepatitis B virus RNA was detected in 9 of 13 male HBsAg-positive/female HBsAg-negative couples (and in 39 of 52 embryos) and 8 of 17 male HBsAg-negative/female HBsAg-positive couples (and in 30 of 63 oocytes and embryos). The HBsAg, which is present in the nuclei and cytoplasm of oocytes and embryos, was detected in 6 of 10 male HBsAg-negative/female HBsAg-positive couples (and in 13 of 20 oocytes and embryos). Hepatitis B virus DNA, HBV RNA, and HBsAg were not found in 135 oocytes and embryos from 39 seronegative couples. Conclusion(s): The presence of HBV in oocytes and embryos suggests the possibility of vertical transmission of HBV via the germ line. (Fertil Steril (R) 2011;95:1667-71. (C)2011 by American Society for Reproductive Medicine.)