The prognostic value of Body Mass Index (BMI) on breast cancer outcome is controversial and previous studies from this unit have not shown any significant relation to survival. The aim of this study was to re-examine any impact of a raised BMI on recurrence and survival related to age and disease stage at the time of diagnosis. Breast cancer patients (2,298) were reviewed and divided in groups by BMI. Recurrence Free Survival (RFS), Breast Cancer Specific Survival (BCSS), and Overall Survival (OS) were compared by Kaplan-Meier life table analysis. Known prognostic factors including BMI were tested for independent prognostic significance in a Cox's regression model. Obese patients (417) had on average larger tumors (median 2.3 versus 2.1 cm, p < 0.01). A trend to an increased positive node status (37% versus 33%) was not significant, p = 0.18. Seven-year RFS was 82% versus 77% in the obese, p < 0.01, BCSS was 87% versus 85%, p = 0.046 and OS 81% versus 77%, p = 0.02. BMI was independently associated with RFS in multivariate analysis (HR: 1.43, p < 0.01). In subgroup analysis, survival differences were most prominent in patients with node positive disease and in patients <60-years old. Breast cancer outcome was worse in patients with a raised BMI and this risk was greater in younger patients and in those with node positive disease. The difference may be related to diagnosis at a more advanced stage in the obese but there was also an independent effect of BMI on survival.