Objectives: Assess population attributable fractions (PAFs) for late postnatal transmission (LPT) of HIV-1 in a cohort of HIV-1-exposed infants. Methods: We used data established from a risk factor analysis of LPT (negative HIV-1 results through the 4-6 week visit, but positive assays thereafter through the 12-month visit) from a perinatal clinical trial conducted in 3 sub-Saharan countries. PAFs were calculated as the proportions of excess LPTs attributed to identified risk factors. Results: For the cohort of 1317 infants, 206 (15.6%) had only low maternal CD4+ counts (<200 cells/mm3), 332 (25.2%) had only high maternal plasma viral loads (VLs) (>50,000 copies/mL), and 81 (6.2%) had both low CD4+ counts and high VLs. Their PAFs were 26.0% [95% confidence interval (CI): 12.0% to 36.0%], 37.0% (95% CI: 22.0% to 51.0%), and 16.0% (95% CI: 6.0% to 25.0%), respectively. Conclusions: Our PAF analysis illustrates the public health impact of the substantial proportion of LPTs accounted for by high-risk women with both low CD4+ counts and high VLs. In light of these results, access to and use of antiretroviral therapy by high-risk HIV-1-infected pregnant women is essential. Additional strategies to reduce LPT for those not meeting criteria for antiretroviral therapy should be implemented.