A pregnant woman with secondary syphilis received appro¬priate therapy with penicillin in the last trimester of preg¬nancy. At delivery, her titre had fallen fourfold, and the baby had a non-reactive cord blood titre at birth. However, the treatment failed to prevent infection in the infant, and the baby had developed signs of congenital syphilis at 10 weeks of age. The definition of “adequate therapy” of pregnant women is unclear, and recent guidelines are contradictory. Therefore, literature that pertains to penicillin therapy in pregnancy is reviewed, and new guidelines for therapy proposed.