Increased body mass index (BMI) at diagnosis has been shown to be associated with an increased risk of disease recurrence and death. However, the association has not been consistent in the literature and may depend on several factors such as menopausal status, extent of disease, and receptor status. We performed a secondary analysis on what we believe is the largest prospective trial of adjuvant chemotherapy to assess the effect of BMI on prognosis in women with lymph node positive breast cancer. The study included 636 women with a median follow-up of over 13 years. Cox's proportional hazards regression model was used to assess the effect of BMI on outcomes. Kaplan-Meier methods were used to estimate survival curves and log rank tests were used to assess differences in survival for BMI groups. We found that increased BMI was generally predictive of faster time to recurrence and decreased survival, but that the relationship was stronger for younger women, those with progesterone receptor negative disease and those with a greater number of lymph nodes that were positive.