Dr. Seth Welles of the University of Minnesota, and his collaborators on the Women-Infant Transmission Study (WITS) presented data on the viral isolates from 136 women who were treated with AZT during pregnancy to estimate the frequency of developing HIV resistance mutations and transmitting resistant virus to their offspring. About a third of the women had been on AZT prior to pregnancy, the median CD4 count for the group was 315 cells/ml3, and the median viral load was 24,800 RNA copies/ml. 25% had at least one AZT resistance mutation, which was asssociated with having a lower CD4 cell count or %, or higher peripheral viral load at the time of delivery. Multivariate analysis suggested that having a reverse transcriptase enzyme mutation, prolonged rupture of the membranes, or a higher peripheral blood lymphocyte count were each independently associated with maternal transmission of HIV. The mother's viral load or CD4 count during pregnancy were not associated with HIV transmission. These findings suggest that in women with prior AZT experience who develop resistant strains and become pregnant and continue AZT monotherapy, the risk of transmitting HIV to their offspring is particularly high. The risk of transmitting HIV was not associated with HIV-related immunosuppression in this study.