We investigated transmission of human immunodeficiency virus type 1 (HIV-1) via breast-feeding by 104 Chinese mothers who acquired the infection through blood transfusion postnatally. Of 106 children, 38 (35.8%) were infected. All children survived to age 5 years, and their survival curve was similar to that of their mothers. These findings suggest a high rate of HIV-1 transmission via breast-feeding when mothers were infected postnatally via blood transfusion, perhaps because of the higher viremia expected during the acute phase of infection. The course of disease among infected children was significantly less rapid than that among newborns infected perinatally, suggesting that a brief window of HIV-1-free life often enables the immune system of an infant to stave off rapid disease progression.