A few lifestyle characteristics before cancer diagnosis have been suggested to modify the prognosis of breast cancer. Follow-up information from 1,453 women with incident invasive breast cancer, diagnosed between 1991 and 1994 and interviewed within the framework of an Italian multicenter case-control study, was used to assess the effect of obesity and of a large spectrum of other factors on breast cancer mortality. Five hundred and three deaths, including 398 breast cancer deaths, were identified. Hazard ratios (HR) for all-cause and breast cancer mortality and corresponding 95% confidence intervals (CI), were calculated using Cox proportional hazards models and adjusted for age and breast cancer characteristics (stage and receptor status). Increased risk of death for breast cancer emerged for body mass index (BMI) ≥ 30 kg/m2 (HR = 1.38; 95% CI: 1.02–1.86), compared to <25, or waist-to-hip ratio (WHR) ≥ 0.85 (HR = 1.27; 95% CI: 0.98–1.64), compared to <0.80, and the strongest association was observed for women with BMI ≥30 and high WHR (≥0.85), compared to women with BMI <25 and WHR < 0.85 (HR = 1.57, 95% CI: 1.08–2.27). The unfavorable effect of high BMI was similar in women <55 and ≥55 years of age, whereas it was stronger in women with I–II stage than III–IV stage breast cancer. Low vegetable and fruit consumption and current or past smoking were also associated to marginally worse breast cancer survival. No significant relationship with survival after breast cancer emerged for several other major lifestyle factors, including physical activity, alcohol drinking, exogenous hormones use and fat intake. High BMI was the lifestyle risk factor that most consistently modified breast cancer prognosis in our study. © 2008 Wiley-Liss, Inc.