1000 pairs of maternal and cord blood samples were collected simultaneously at the time of delivery. 23 (2.3%) of the maternal samples were positive for HBsAg by enzyme-linked immunosorbent assay. HBeAg was detected in 11 (47%) of the 23 HBsAg positive mothers and anti-HBeAg was detected in another 5 samples. HBsAg and HBeAg were detected in 7 (30%) of the 23 cord blood samples from HBsAg-positive mothers, and anti-HBeAg was detected in one of these samples. At follow-up (6-18 months), antigenaemia had persisted in 17 (85%) of the 20 HBsAg-positive mothers and in 9 (45%) of 20 babies born to HBsAg-positive mothers. Seven of the 10 babies (70%) born to mothers positive for both HBsAg and HBeAg had persistent HBsAg in their blood, in contrast to 2 of the 10 babies (20%) born to mothers positive for HBsAg only. However, none of these mothers or their babies were found to have anti-HBeAg at follow-up. We conclude that the presence of HBeAg in mothers' blood enhances vertical transmission of hepatitis B virus infection to their babies.