BACKGROUND: Obese breast cancer patients have a poorer prognosis than non-obese patients. We examined data from a large prospective cohort study to explore the associations of obesity with tumour pathology, treatment and outcome in young British breast cancer patients receiving modern oncological treatments. PATIENTS AND METHODS: A total of 2956 patients aged ≤40 at breast cancer diagnosis were recruited from 126 UK hospitals from 2001 to 2007. Height and weight were measured at registration. Tumour pathology and treatment details were collected. Follow-up data were collected at 6, 12 months, and annually. RESULTS: A total of 2843 eligible patients (96.2%) had a body mass index (BMI) recorded: 1526 (53.7%) were under/healthy-weight (U/H, BMI