The prevention of mother to child transmission of HIV-1 by zidovudine monotherapy is well known, but increasingly combination anti-retroviral therapy is prescribed during pregnancy. In this prospective study, 19 pregnant women with human immunodeficiency virus-1 (HIV-1) infection who elected to take anti-retroviral therapy during the second and third trimesters were treated with zidovudine or zidovudine plus lamivudine. Fourteen women treated with zidovudine monotherapy had a mean 0.3 log(10) reduction in viral load and a mean 52 x 10(6)/L (17%) increase in CD4+ lymphocytes at delivery compared with pre-treatment samples. Genotypic mutations associated with decreased susceptibility to zidovudine were detected in 2 of 10 women at delivery. Five women with more advanced HIV-1 infection were treated with zidovudine plus lamivudine and a mean 1.5 log(10) reduction in viral load together with a mean 30 x 10(6)/L (33%) increase in CD4+ lymphocytes was observed in this group. However, four of five women in the dual therapy arm had the M184V mutation in the reverse transcriptase gene associated with decreased susceptibility to lamivudine at delivery. We conclude that zidovudine plus lamivudine reduced HIV-1 plasma viraemia to low levels in pregnant women with advanced HIV-1 disease but the rapid development of genotypic resistance to lamivudine indicates that additional therapy is required both for the long-term benefit of the mothers and to prevent the development of resistant virus that may be transmitted to the infant.