Objectives To test the hypothesis that duration of breast feeding is related to changes in vascular function relevant to the development of cardiovascular disease. Design Population based observational study, Setting Cambridge. Participants 331 adults 171 women, 160 men) aged between 20 and 28 years, born in Cambridge Maternity Hospital. Main outcome measures Distensibility of brachial artery, type and duration of infant feeding, current lipid profile, and other cardiovascular. risk factors. Results The longer the period of breast feeding the less distensible the artery wall in early adult life, with no ses differences (regression coefficient = - 3.93 mum/month, 95% confidence interval - 7.29 to - 0.57, P = 0.02), However, in those breast fed for less than four months, arterial distensibility was not significantly reduced compared with an exclusively formula fed group. The vascular changes observed were not explained by alterations in plasma cholesterol concentration in adult life. Conclusions Breast feeding in infancy is related to reduced arterial function 20 years later. These data should not alter current recommendations in favour of bl east feeding, which has several benefits for infant health. Further work is needed, however, to explore the optimal duration of breast feeding in relation to cardiovascular outcomes.