Objectives. We investigated the relationship between breastfeeding, asthma and atopy, and child body mass index (BMI).Methods. From a prospective birth cohort (n = 2860) in Perth, Western Australia, 2195 children were followed up to age 6 years. Asthma was defined as doctor-diagnosed asthma and wheeze in the last year, and atopy was determined by skin prick test of 1596 children. Breastfeeding, BMI, asthma, and atopy were regressed allowing for confounders and the propensity score for overweight.Results. Using fractional polynomials, we found no association between breastfeeding and overweight. Less exclusive breastfeeding was associated with increased asthma and atopy, and BMI increased with asthma.Conclusions. Less exclusive breastfeeding leads to increases in child asthma and atopy and a higher BMI is a risk factor for asthma.