In 1994, zidovudine (ZDV) was demonstrated to substantially reduce perinatal transmission of the human immunodeficiency virus (HIV). Guidelines regarding the use of ZDV to reduce transmission and regarding counseling and voluntary testing of pregnant women were issued in 1994 and 1995, respectively. Surveillance methods were used to evaluate the implementation of these guidelines and to understand reasons for continued perinatal transmission of HIV. Population-based enhanced perinatal surveillance was used in seven states to collect information regarding mother-infant pairs in 1993, 1995, and 1996. Birth registries and HIV/Acquired immunodeficiency virus (AIDS) registries were matched to determine the number of HIV-infected women with diagnosis before delivery. Supplemental epidemiologic information was collected for 1,321 pairs. The estimated total number of HIV-infected women giving birth each year was derived from the Survey of Childbearing Women, an anonymous serologic survey of the prevalence of HIV infection among women giving birth. From 1993 through 1996, the proportion of HIV-infected women with diagnosis before delivery increased from 70% to 80%. The proportion of women with a diagnosis who received ZDV prenatally increased from 27% to 83% and intrapartum, 6% to 75%; for neonates, the increase was from 8% to 77%. Overall, 14% of women received no or only one prenatal care visit. A total of 36% of women who used illicit drugs during pregnancy had not had prenatal care. Of the children who received any ZDV, 8% were infected compared with 16% of those who received no ZDV. ZDV, used for treating pregnant HIV-infected women, has been rapidly adopted in clinical practice and has reduced the transmission of HIV. To achieve continued declines in perinatal transmission of HIV infection, continued progress is needed in the following areas: a) increases in the proportion of women who receive prenatal care and an HIV diagnosis; and b) implementation of rapid testing methods (when licensed rapid tests are available) or rapid turnaround of standard tests (expedited EIA tests).