PURPOSE: Adolescent females aged 15-19 account for 62% of new HIV infections and give birth to 16 million infants annually. We quantify the risk of early mother-to-child transmission (MTCT) of HIV among adolescents enrolled in nationally representative MTCT surveillance studies in South Africa. METHODS: Data from 4,814 adolescent (≤19 years) and 25,453 adult (≥20 years) mothers and their infants aged 4-8 weeks were analyzed. These data were gathered during three nationally representative, cross-sectional, facility-based surveys, conducted in 2010, 2011-2012, and 2012-2013. All infants were tested for HIV antibody (enzyme immunoassay), to determine HIV exposure. Enzyme immunoassay-positive infants or those born to self-reported HIV-positive mothers were tested for HIV infection (total nucleic acid polymerase chain reaction). Maternal HIV positivity was inferred from infant HIV antibody positivity. All analyses were weighted for sample realization and population live births. RESULTS: Adolescent mothers, compared with adult mothers, have almost three times less planned pregnancies 14.4% (95% confidence interval [CI]: 12.5-16.5) versus 43.9% (95% CI: 42.0-45.9) in 2010 and 15.2% (95% CI: 13.0-17.9) versus 42.8% (95% CI: 40.9-44.6) in 2012-2013 (p < .0001), less prevention of MTCT uptake (odds ratio [OR] in favor of adult mothers = 3.36, 95% CI: 2.95-3.83), and higher early MTCT (adjusted OR = 3.0, 95% CI: 1.1-8.0), respectively. Gestational age at first antenatal care booking was the only significant predictor of early MTCT among adolescents. CONCLUSIONS: Interventions that appeal to adolescents and initiate sexual and reproductive health care early should be tested in low- and middle-income settings to reduce differential service uptake and infant outcomes between adolescent and adult mothers.