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Abstract: Body mass index (BMI) is a major risk factor for endometrial cancer incidence but its impact on post-treatment survival is unclear. We investigated the relationships of BMI (categorised using the WHO definitions) with clinico-pathological characteristics and outcome in women treated within the MRC ASTEC randomised trial, which provides data from patients who received standardised allocated treatments and therefore reduces biases. The impact of BMI on both recurrence-free survival (RFS) and overall survival (OS) was analysed using the Cox regression models. An a priori framework of evaluating potential biases was explored. From 1408 participants, there were 1070 women with determinable BMI (median=29.1kg/m2). Histological types were endometrioid (type 1) in 893 and non-endometrioid (type 2) in 146 women; the proportion of the latter decreasing with increasing BMI (8% versus 19% for obese III WHO category versus normal weight, p trend =0.003). For type 1 carcinomas, increasing BMI was associated with less aggressive histopathological features (depth of invasion, p =0.006; tumour grade, p =0.015). With a median follow-up of 34.3months, there was no influence of BMI on RFS - adjusted HRs per 5kg/m2 were 0.98 (95% CI 0.86, 1.13) and 0.95 (0.74, 1.24), for type 1 and 2 carcinomas; and no influence on OS – adjusted HRs per 5kg/m2 were 0.96 (0.81, 1.14) and 0.92 (0.70, 1.23), respectively. These findings demonstrate an important principle: that an established link between an exposure (here, obesity) and increased incident cancer risk, does not necessarily translate into an inferior outcome following treatment for that cancer. [Copyright &y& Elsevier]