Pregnant women with untreated syphilis have high rates of adverse pregnancy outcome. Early syphilis in pregnancy, if not treated, results in 40% loss either by spontaneous abortions, stillbirth, or perinatal death. Another 40% of such pregnancies result in an infant born with congenital syphilis. Only 20% of such pregnancies result in a normal infant. Currently recommended treatment of early syphilis in pregnant women uses a dosage of 2.4 million U of benzathine penicillin G. Because of the potentially disastrous effect syphilis has on pregnancy outcome, the occurrence of any treatment failure must be considered seriously. In this brief report we present clinical summaries of four pregnant women treated for syphilis who represent probable treatment failures.