Higher body mass index (BMI) measured before endometrial cancer diagnosis has been associated with greater risk of developing endometrial cancer and higher mortality, but the association between BMI measured after diagnosis and mortality risk is unclear. We identified 467 women (91 deaths) in the Women’s Health Initiative (WHI) with information on BMI measured after diagnosis and used Cox proportional hazards regression to generate hazard ratios (HR) and 95% confidence intervals (CI) for all-cause mortality. Comparing BMI 35+ with <25 kg/m$^2$, we observed no association with all-cause mortality (HR = 1.02, 95% CI 0.55–1.91). Our study does not support the hypothesis that higher BMI after endometrial cancer diagnosis is associated with poorer survival.