Background: Using adequate infection control measures, the rate of vertical transmission of human immunodeficiency virus (HIV) during pregnancy, has been reduced to 3% in Chile. Aim: To determine vertical transmission rate and risk factors associated to perinatal infection in pregnant women with known (KI) and unknown HIV infection (UI). Patients and methods: HIV infected pregnant women whose deliveries were attended at the San Borja Arriaran Hospital were included. Antiretroviral therapy (ART) has been used since 1995 (Zidovudine 13 patients, bitherapy 4 and triple therapy 14 patients). Newborns have received ART since 1995. Premature labor without evident cause, premature rupture of membranes, and rupture of membranes over 4 h before delivery were evaluated. Delivery was by elective cesarean section since 1993. Breast feeding was avoided. Pregnant women with UI (suspected disease after delivery due to child or mother pathology) did not received ART. Delivery and breast feeding were managed with common obstetrical-neonatal criteria. Results: Fifty three HIV infected pregnant women were studied (43 with KI and 10 with UI). Four children (36.4%) from the KI group and seven (63.6%) from the UI group became infected. The global rate of vertical transmission among KI group was significantly lower than UI group: 9.5% (4/42) vs 70.0% (7/10) p <0.001. Using ART, this rate was further reduced to 6.5% (2/31) and with bitherapy or triple therapy to 0% (0/18). Breast feeding, vaginal delivery, premature delivery with no clinical cause, premature rupture of membranes, rupture of membranes longer than 4 h and lack of ART, were significantly more common in the UI group, compared with KI group. Conclusions: Vertical transmission in pregnant women with KI is significantly lower compared with UI. Risk factors increasing HIV perinatal infection are: breast feeding, lack of ART, vaginal delivery, premature rupture of membranes, rupture of membranes >4 h and premature labor without a clinical cause (Rev Méd Chile 2003; 131: 633-640)