The purpose of this study was to characterize pregnancies that were complicated by maternal syphilis that had been treated before delivery in which the newborn infant was diagnosed with congenital syphilis. Prospective surveillance from January 1, 1982, to December 31, 1998, involved women who received antenatal treatment for syphilis. Infants who were born with congenital syphilis were identified by clinical or laboratory criteria. Antepartum factors such as gestational age, time to delivery and VDRL titers were then analyzed and compared with those of women who had been treated and who were delivered of an uninfected infant. The 1:1 match was based on the stage of syphilis and the gestational age at treatment. Forty-three women who received antepartum therapy for syphilis were delivered of an infant with congenital syphilis. Most of the women had been treated for early syphilis; the mean gestational age at treatment was 30.3 weeks. Thirty-five percent of the women were treated >30 days before delivery. Fifty-six percent of the infants were preterm. The 1:1 match revealed that treatment and delivery high VDRL titers, prematurity, and a short interval from treatment to delivery were significantly different in those infants who were diagnosed with congenital syphilis. High VDRL titers at treatment and delivery, earlier maternal stage of syphilis, the interval from treatment to delivery, and delivery of an infant at < or =36 weeks' gestation are associated with the delivery of a congenitally infected neonate after adequate treatment for maternal syphilis.