In low-income settings, human immunodeficiency virus (HIV)-positive mothers must choose between breastfeeding their infants and risking transmission of HIV or replacement feeding their infants and risking diarrheal disease from contaminated water. We conducted a cross-sectional study of children < 2 years of age of 254 HIV-positive mothers in peri-urban Zambia to assess their exposure to waterborne fecal contamination. Fecal indicators were found in 70% of household drinking water samples. In a multivariable analysis, factors associated with diarrhea prevalence in children < 2 years were mother having diarrhea (adjusted odds ratio [aOR] = 5.18, 95% confidence interval [CI] = 1.65-16.28), child given water in the past 2 days (aOR = 4.08, 95% CI = 1.07-15.52), child never being breastfed (aOR = 2.67, 95% CI = 1.06-6.72), and rainy (versus dry) season (aOR = 4.60, 95% CI = 1.29-16.42). Children born to HIV-positive mothers were exposed to contaminated water through direct intake of drinking water, indicating the need for interventions to ensure microbiological water quality.