Aims This study explores associations between early weaning and alcohol use disorders in youth and mechanisms by which these associations may operate. Design We used data from the Mater University Study of Pregnancy and its outcomes, an Australian birth cohort study based in Brisbane. Setting and participants: This study is based on a subsample of 2370 participants for whom complete data were available at age 21 years. Length and method of breastfeeding were assessed at 6 months. Measurements Alcohol use disorders were assessed at age 21 using the life-time version of the Composite International Diagnostic Interview—computerized version (CIDI-Auto). We adjusted for maternal age, marital status, education, alcohol, tobacco use, anxiety, depression and maternal attitudes towards the baby. Attention Deficit and Hyperactivity Disorders (ADHD) and Intellect Quotient (IQ) were measured with the Child Behaviour Checklist (5 years) and the Ravens SM (14 years), respectively. Findings Those who had been weaned within 2 weeks of being born and breastfed at regular intervals were at increased risk of meeting criteria for alcohol use disorders at age 21 [odds ratio (OR) 1.71, 95% confidence interval (CI):1.07, 2.72]. Conclusion This study confirms a small but robust association between early weaning and increased risk of alcohol use disorders.