To assess the role of maternal viraemia in vertical transmission of HIV and the extent to which viraemia occurs during the various stages of pregnancy, we have attempted to detect human immunodeficiency virus (HIV) in 44 pregnant HIV-1 infected women during 47 pregnancies (30 continued, 17 aborted) and in 30 children and 12 fetuses. Infectious HIV was detected at some time during pregnancy in 59% of women from plasma and in 83% from either peripheral blood mononuclear cells or plasma. HIV was not isolated from any of the newborn babies (0/27) at birth. The mothers had a significantly higher frequency of viraemia during pregnancy than their children had by 6 months of age (p = 0.002); by this time HIV was recovered from 5 (26%) of 19 infants. HIV was not detected by virus isolation, in-situ hybridisation, or polymerase chain reaction (PCR) in 10 fetuses; the other 2 fetuses were positive either by in-situ hybridisation or by PCR but neither result could be confirmed in a second organ or by the other methods of detection. The findings show that there is no consistent spread of HIV across the placenta during maternal viraemia, and indicate that in most cases transmission occurs close to or at delivery.