A regulation mandating a syphilis serology test at delivery was implemented on December 6, 1989. To assess any impact on congenital syphilis reporting, we compared all cases reported in Upstate New York for the year directly preceding the regulation (n = 69) to those born in the three-year period following its implementation (n = 239). After implementation, the percentage of cases not tested at delivery decreased from 10 percent to 1 percent and reports of syphilitic stillbirths tripled. Asymptomatic infection of both mothers and babies increased significantly and at-birth detection of cases in women with negative prenatal serologies increased by 43 percent. Delivery screening failed to identify seven cases due to incubating maternal infection.