Vertical transmission of hepatitis B virus (HBV) was studied in 21 mother-infant pairs. A transient HBs-antigenemia was observed in 60% of the 10 children born to HBsAg healthy carrier mothers and 91% of the 11 babies born to mothers with HBsAg-positive CAH. Among the 16 children showing HBs-antigenemia, 3 developed acute viral hepatitis two of whom showed progression to chronicity.