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To determine the rates of, and risk factors for, mother-to-child transmission (MCT) of HIV-1 and HIV-2 infection in The Gambia.A blinded, prospective, community-based cohort study of 29 549 pregnant women attending the eight largest antenatal clinics in The Gambia.Women were tested for HIV-1 and HIV-2 infection. Infected subjects and a group of HIV-seronegative women were followed with their babies until 18 months after delivery. Maternal CD4 cell count percentages were measured before and 18 months after delivery, and the antenatal plasma viral load was determined. Babies were tested for HIV by the polymerase chain reaction and/or serology at 2, 9 and 18 months of age.The study enrolled 144 women positive for HIV-1 and 294 for HIV-2 plus 565 seronegative pregnant women: the mean antenatal percentage CD4 cell counts of 96 HIV-1-positive, 223 HIV-2-positive and 125 HIV-seronegative mothers were 31% [95% confidence interval (CI) 28–33], 41% (95% CI 39–42) and 47% (95% CI 45–49), respectively. The geometric mean antenatal plasma viral load of 94 HIV-1-infected women was 15 100 copies×103ml (95% CI 10 400–19 000) which was much higher than that of 60 randomly selected HIV-2-infected women, which was 410 copies×103ml (95% CI 150–910) (P < 0.001). The estimated transmission rate of HIV-1 was 24.4% (95% CI 14.6–33.9) and that of HIV-2 was 4.0% (95% CI 1.9–7.4). Five of 17 HIV-1-positive and three of eight HIV-2-positive babies were infected after 2 months of age. Birth in the rainy season [odds ratio (OR) 2.9; 95% CI 1.2–7.2], a low postnatal CD4 cell percentage (OR for a 10% fall 2.4; 95% CI 1.1–5.1) and a high maternal plasma viral load (OR for a 10-fold increase 2.9; 95% CI 1.1–7.8) were risk factors for transmission that applied equally to both viruses.Low maternal HIV-2 RNA levels, which on average are 37-fold less than in HIV-1 infection, relate to the low MCT rate of HIV-2.