In this study it has been evaluated the connection between the maternal immune status and the obstetrical factors on the one hand end the perinatal transmission of HIV in the other as well as, the effects of zidovudine prophylaxis and elective caesarean section on vertical HIV transmission. From January 1987 to September 1999, 60 HIV infected pregnant women were followed with their 64 infants. From July 1995 15 women and their respective children were treated with zidovudine prophylaxis (protocol ACTG 076); from June 1997, in 8 of these patients a systematic caesarean section was performed (7 electively and 1 on an emergent basis). The transmission rare was 20.5% among the 44 children in the group without zidovudine prophylaxis, compared with 7.1% among the 14 children in the group receiving Zidovudine. As for as the mother-child pairs receiving Zidovudine therapy, the rate of vertical transmission was 0% for the 7 mothers who underwent elective cesarean section and 14.3% for the 7 mothers with other ways of delivery. The interaction between zidovudine prophylaxis and elective caesarean section was associated with the lowest rate of vertical HIV transmission.