Obese and underweight women who develop breast cancer may have poorer survival compared with normal-weight women. However, the optimal weight for best prognosis is still under study. We conducted a prospective investigation of pre-diagnosis body mass index (BMI) and mortality among 14,948 breast cancer patients in the After Breast Cancer Pooling Project. Breast cancer patients diagnosed from 1990 to 2006 with AJCC Stage I-III breast tumors were drawn from four prospective cohorts. Hazard ratios (HR) and 95% confidence intervals (CI) representing the associations of BMI categories (World Health Organization international classifications) with recurrence and mortality were estimated using delayed entry Cox proportional hazards models. Obese (30 to <35 kg/m(2)), severely obese (35 to <40 kg/m(2)), and morbidly obese (>/=40 kg/m(2)) were examined. After a mean follow-up of 7.8 years, 2,140 deaths and 2,065 recurrences were documented. Both underweight (HR = 1.59; 95% CI: 1.18, 2.13) and morbidly obese women (HR = 1.81; 95% CI: 1.42, 2.32) had the greatest risk of overall mortality compared with normal weight (18.5-24.9 kg/m(2)) women. Severe obesity (HR = 1.09; 95% CI: 0.88, 1.36) and obesity (HR = 1.11; 95% CI: 0.97, 1.27) were related to small non-significant increased risks. Overweight (25.0-29.9 kg/m(2)) was not associated with any excess risk compared with normal weight. Similar associations were found for breast cancer death and non-breast cancer death but not recurrence. Women who were underweight and morbidly obese before breast cancer diagnosis were at the greatest risk of all-cause mortality. Morbidly obese women were also at increased risk of death from breast cancer. These results suggest that degree of obesity confers differential risk on survival.