In most obstetric centers routine serum screening for hepatitis B is not part of standard prenatal care. This study was designed to determine whether hepatitis B screening is cost effective for routine prenatal testing. In a prenatal population of 585 adolescents the cost-benefit ratio of hepatitis B screening was compared with that of routine syphilis screening. Eight positive results were detected, with a cost of $1755 per positive case. This compared favorably with the frequency and cost of detection of syphilis in the same population. Routine hepatitis B screening is recommended for consideration in similar prenatal populations.