Of 4588 pregnant women in a high-risk Haitian population, 443 (9.7%) were serologically positive for the human immunodeficiency virus type 1 (HIV-1). Infants born to women who were HIV-1 seropositive were more likely to be premature, of low birth weight, and malnourished at 3 and 6 months of age than were infants born to women who were HIV-1 seronegative. Increased mortality was observed in infants born to women who were HIV-1 seropositive by 3 months of age. At 12 months of age, 23.4% of the infants born to women who were HIV-1 seropositive had died compared with 10.8% of the infants born to women who were HIV-1 seronegative; at 24 months of age, the mortality rates were 31.3% and 14.2%, respectively. Maternal HIV-1 infections resulted in an 11.7% increase in the overall infant mortality rate in this population. The estimated mother-to-infant HIV-1 transmission rate in these breast-fed infants was 25%, similar to the rates reported for non-breast-fed populations in the United States and Europe.