BACKGROUND AND OBJECTIVES: Although prenatal alcohol and nicotine exposure are associated with reduced cognition in children, associations between consumption of alcohol during lactation and cognition have not been examined. We aimed to examine whether drinking or smoking while breastfeeding lowers children’s cognitive scores. We hypothesized that increased drinking or smoking would be associated with dose-dependent cognitive reductions. METHODS: Data were sourced from Growing Up in Australia: The Longitudinal Study of Australian Children. Participants were 5107 Australian infants recruited in 2004 and assessed every 2 years. Multivariable linear regression analyses assessed relationships between drinking and smoking habits of breastfeeding mothers and children’s Matrix Reasoning, Peabody Picture Vocabulary Test–Third Edition and Who Am I? scores at later waves. RESULTS: Increased or riskier wave 1 maternal alcohol consumption was associated with reductions in Matrix Reasoning scores at age 6 to 7 years in children who had been breastfed (B = −0.11; SE = 0.03; 95% confidence interval: −0.18 to −0.04; P = .01). This relationship was not evident in infants who had never breastfed (B = −0.02; SE = 0.10; 95% confidence interval = −0.20 to 0.17; P = .87). Smoking during lactation was not associated with any outcome variable. CONCLUSIONS: Exposing infants to alcohol through breastmilk may cause dose-dependent reductions in their cognitive abilities. This reduction was observed at age 6 to 7 years but was not sustained at age 10 to 11 years. Although the relationship is small, it may be clinically significant when mothers consume alcohol regularly or binge drink. Further analyses will assess relationships between alcohol consumption or tobacco smoking during lactation and academic, developmental, physical, and behavioral outcomes in children.