Background: The importance of plasma levels of human immunodeficiency virus type 1 (HIV-1) RNA in pregnant women in relation to the other factors known to influence the risk of transmission of infection to their infants is incompletely defined. We studied the relation of maternal plasma HIV-1 RNA levels to the risk of perinatal transmission and the timing of transmission. Methods: We measured plasma HIV-1 RNA serially in 552 women with HIV-1 infection who had singleton pregnancies. The status of infection in their infants was assessed by culture of blood and further classified as early (if a culture of blood obtained within the first two days of life was positive) or late (if a culture of blood obtained in the first seven days of life was negative but subsequent cultures were positive). The rates of transmission at various levels of maternal plasma HIV-1 RNA were analyzed by tests for trend, with adjustment for covariates by stratification and logistic regression. Results: Increasing geometric mean levels of plasma HIV-1 RNA were associated with increasing rates of transmission: the rate was 0 percent among women with less than 1000 copies per milliliter (0 of 57), 16.6 percent among women with 1000 to 10,000 copies per milliliter (32 of 193), 21.3 percent among women with 10,001 to 50,000 copies per milliliter (39 of 183), 30.9 percent among women with 50,001 to 100,000 copies per milliliter (17 of 54), and 40.6 percent among women with more than 100,000 copies per milliliter (26 of 64, P<0.001). The treatment status of one woman was unknown. The highest rate of transmission was among women whose plasma HIV-1 RNA levels exceeded 100,000 copies per milliliter and who had not received zidovudine (19 of 30 women, 63.3 percent). Neither higher HIV-1 RNA levels early in pregnancy nor higher levels late in pregnancy were associated with the timing of infection in the infants. Conclusions: In pregnant women with HIV-1 infection, the level of plasma HIV-1 RNA predicts the risk but not the timing of transmission of HIV- 1 to their infants.