PIP: The significance of understanding the vertical transmission of HIV lies in the 78% of women's AIDS cases (1991) of sexually active women who may transmit the infection to their offspring. 2% of children in the US and 5-25% of children in developing countries are HIV infected. The current rates of maternal-fetal transmission average 25-30%. Interrupting this mode of passage would provide protection for children. The Van de Perre study on postnatal transmission by breast feeding is discussed in order to the timing of HIV infection and the relationship to infant risk of infection. Postnatal infection of infants suggests that the risk is enhanced during the primary infection of the mother. It is pointed out that transient but large elevations in plasma virus titers have been observed during primary infection of homosexual men, before anti-p24 and anti-gp160 antibodies appear. These plasma titers may increase the efficiency of vertical transmission. It is also possible that the absence of neutralizing or nonneutralizing antibodies may enhance transmission. The frequency of new infections is estimated by Van de Perre in the rate of seroconversion in Rwandan women as 4.7-7.3%/year. The US rate is .6-.8/1000/year for all adults and adolescents. Pregnancy can occur after primary HIV infection or at the onset when plasma titers are high and antibody levels are low. It is reported that the rate of transmission may be highest when mothers are in a more advanced disease state. Another factor affecting transmission is the presence or absence of maternal antibodies. Lower HIV transmission may be related to the presence of epitopes on the hypervariable V3 loop of HIV gp120 or the principal neutralizing domain. Although HIV virus has been identified in fetal tissues after 8-15 weeks gestation, there is reason to believe that the greatest transmission is intrapartum because maternal blood may be ingested or there is maternal-fetal transfusion during labor and delivery. New York studies have suggested that 50% of infected infants of known seropositive women will be positive by the polymerase chain reaction by 3 months of age and 50% by 3-6 months of age. It is likely that the delay is due to infection late in gestation.