OBJECTIVE: We examined the association between body mass index (BMI) around the time of diagnosis, weight change post-diagnosis, and breast cancer prognosis in a prospective cohort study of 1,692 breast cancer survivors. METHODS: Pre-diagnosis weight, weight at study entry, and height was obtained from mailed questionnaires and then weight change and BMI were calculated. After approximately seven years of follow-up, 207 recurrences, 99 deaths due to breast cancer, and 162 deaths due to any cause were reported. Delayed entry Cox proportional hazard models were used to estimate hazard ratios (HR) and 95% confidence intervals (CI), controlling for treatment and known prognostic factors. RESULTS: Being obese one year before diagnosis was associated with an increased risk of death from any cause (HR = 1.6; 95% CI: 1.1-2.3) and a suggestion of increased risk of death from breast cancer (HR = 1.6; 95% CI: 0.9-2.7). However, weight gain up to four years after a breast cancer diagnosis was not associated with an increased risk of recurrence or death from any cause nor did moderate weight loss (5-10%) decrease risk of these outcomes. There was some evidence that women who had larger weight losses (>or=10%) between pre-diagnosis and study entry had an increased risk of recurrence (HR = 1.7; 95% CI 1.0-2.6) and death due to any cause (HR = 2.1; 95% CI 1.3-3.4) compared to being weight stable. This elevated risk was more pronounced among women who were obese before diagnosis (BMI >or= 30 kg/m(2)) or who had ER- or PR- tumors. CONCLUSION: We found that being obese before breast cancer diagnosis was associated with increased risk of recurrence and poorer survival, corroborating results from previous studies. However, weight gain after diagnosis did not confer additional risk. Body weight pre-diagnosis appears to be the strongest predictor of an adverse breast cancer prognosis.