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BACKGROUNDBody Mass index (BMI) has been shown to affect risk and mortality of several cancers. Prostate cancer and obesity are major public health concerns for middle-aged and older men. Previous studies of pre-diagnostic BMI have found an increased risk of prostate cancer mortality in obese patients. OBJECTIVETo study the associations between BMI at time of prostate cancer diagnosis and prostate cancer specific and overall mortality. METHODSBMI was analyzed both as a continuous variable and categorized into four groups based on the observed distribution in the cohort (BMI<22.5, 22.5<25, 25<27.5 and 27.5kg/m(2)). The association between BMI and mortality was assessed using stratified Cox proportional hazards models and by fitting regression splines for dose response analysis in 3,161 men diagnosed with prostate cancer. After 11 years of follow up via linkage to the population-based cause of death registry, we identified 1,161 (37%) deaths off which 690 (59%) were due to prostate cancer. RESULTSHigh BMI (BMI27.5kg/m(2)) was associated with a statistically significant increased risk of prostate cancer specific mortality (HR:1.44, 95%CI: 1.09-1.90) and overall mortality (HR:1.33, 95%CI: 1.09-1.63) compared to the reference group (BMI 22.5<25kg/m(2)). Additionally, men with a low BMI (<22.5kg/m(2)), had a statistically significant increased risk of prostate cancer specific mortality (HR:1.33, 95%CI: 1.02-1.74) and overall mortality (HR:1.36, 95%CI: 1.11-1.67) compared to the reference. However, this effect disappeared when men who died within the first two years of follow-up were excluded from the analyses while the increased risk of prostate cancer specific mortality and overall mortality remained statistically significant for men with a BMI27.5kg/m(2) (HR:1.44, 95%CI: 1.09-1.90 and HR: 1.33, 95%CI: 1.09-1.63, respectively). CONCLUSIONThis study showed that a high BMI at time of prostate cancer diagnosis was associated with increased overall mortality. Prostate 75: 1129-1136, 2015. (c) 2015 Wiley Periodicals, Inc.