Amniocentesis was performed under sonographic guidance in gravidas (< 20 weeks' gestation) with untreated syphilis. Five to ten millilitres of amniotic fluid from each patient was used for rabbit infectivity testing (RIT) and polymerase chain reaction (PCR) to detect amniotic fluid infection with Treponema pallidum. Gravidas were treated with benzathine penicillin G. Newborns were examined for clinical and laboratory signs of congenital syphilis including immunoglobulin M (IgM) antibody to T. pallidum by Western blotting (immunoblotting). Eleven patients were enrolled at a mean gestational age of 16.8 weeks. T. pallidum was recovered from amniotic fluid by RIT in four cases (36 per cent), and PCR was positive in three of the amniotic fluid specimens (27 per cent). There were no false-positive PCR results. None of the newborns had clinical evidence of congenital syphilis and their sera lacked IgM reactivity to T. pallidum antigens by immunoblotting. These findings confirm in utero infection with T. pallidum in continuing early pregnancy and indicate that in utero treponemal infection can be eradicated by maternal treatment.