The primary aim was to determine whether there was an inverse association between the duration of exclusive breastfeeding and the development of traits associated with asthma in children at age six years. A prospective cohort study of children from Western Australia was enrolled prior to birth and followed to age six. Two thousand, nine hundred and seventy-nine children were recruited through antenatal clinics at the major tertiary obstetric hospital in Perth. Unconditional logistic regression was used to model the association between duration of exclusive breastfeeding and outcomes related to asthma or atopy at age six allowing for a number of important confounders. These included gender, gestational age, smoking in pregnancy and early child care. After adjustment for confounders, the introduction of milk other than breastmilk before four months of age was a significant (p < 0.05) risk factor for all asthma-related outcomes in six-year-old children: (i) doctor diagnosed asthma odds ratio ¿OR¿ = 1.25 (95% CI 1.02-1.54); (ii) wheeze three or more times since the age of one year OR 1.42 (1.15-1.76); (iii) wheeze in the last twelve months OR 1.28 (1.02-1.76); (iv) sleep disturbance due to wheeze within the last twelve months OR 1.41 (1.04-1.90); (v) age at doctor diagnosis (hazard ratio ¿HR¿ 1.22 ¿1.03-1.43¿); (vi) age at first wheeze (HR 1.36 ¿1.17-1.59¿) and; (vii) positive reaction to common aeroallergens OR 1.27 (1.01-1.59). There is a substantial reduction in risk of childhood asthma as assessed at age six years, if exclusive breastfeeding is continued for at least the first four months of life. These findings are important for our understanding of the aetiology of and for the potential prevention of asthma in children.