Ninety-nine preterm infants with birth weights <1750 gm had three doses of hepatitis B vaccine. Fifty-seven received the first dose when they weighed greater-than-or-equal-to 1000 gm (group 1) and 42 when they weighed greater-than-or-equal-to 2000 gm (group 2). The final seropositive rates and geometric mean titers of group 1 infants (79%, 61 mIU/ml) and group 2 infants (91%, 262 mIU/ml) were less than that of 43 normal term infants (400%, 679 mIU/ml).