OBJECTIVE: To assess the association between fasting insulin levels and breast cancer. DESIGN: Cross sectional study. PARTICIPANTS: 3868 women aged 60-79 years. MAIN OUTCOME MEASURE: Prevalent breast cancer (151 cases). RESULTS: Insulin levels were positively associated with breast cancer. The age adjusted odds ratio (95% confidence interval) for a one unit increase in log(e) insulin levels among women without diabetes was 1.34 (1.02, 1.77). This association was not substantively altered by adjustment for potential confounding factors (age of menopause, hysterectomy/oophorectomy, hormone replacement use, oral contraceptive use, parity, adult social class and smoking) or potential mediating factors (body mass index, waist to hip ratio, leg length, age at menarche and childhood social class). Women with both long legs and higher insulin levels were at particularly increased risk, with breast cancer prevalence being 5.7% among women in the highest thirds of both insulin levels and leg-length compared to 1.8% among those in the lowest thirds of both. Positive associations between insulin levels and breast cancer were found for both pre- and post-menopausal breast cancers. Fasting glucose levels, HOMA score, diabetes and a history of gestational glycosuria or diabetes were also positively associated with breast cancer CONCLUSIONS: Hyperinsulinaemia is positively associated with breast cancer in this cohort of older women. This effect may be mediated via a number of hormonal pathways acting at different stages of the life course.