Background: Many large-scale epidemiological data sources used to evaluate the body mass index (BMI: kg/m2) mortality association have relied on BMI derived from self-reported height and weight. Although measured BMI (BMI M) and self-reported BMI (BMISR) correlate highly, self-reports are systematically biased.Objective:To rigorously examine how self-reporting bias influences the association between BMI and mortality rate. Subjects: Samples representing the US non-institutionalized civilian population.Design and Methods:National Health and Nutrition Examination Survey data (NHANES II: 1976-80; NHANES III: 1988-94) contain BMI M and BMISR. We applied Cox regression to estimate mortality hazard ratios (HRs) for BMI M and BMI SR categories, respectively, and compared results. We similarly analyzed subgroups of ostensibly healthy never-smokers.Results:Misclassification by BMISR among the underweight and obesity ranged from 30-40% despite high correlations between BMI M and BMISR (r0.9). The reporting bias was moderately correlated with BMI M (r0.35), but not BMI SR (r0.15). Analyses using BMISR failed to detect six of eight significant mortality HRs detected by BMI M. Significantly biased HRs were detected in the NHANES II full data set (χ2 12.49; P<0.01) and healthy subgroup (χ22 9.93; P<0.04), but not in the NHANES III full data set (χ22 5.63; P<0.23) or healthy subgroup (χ22 1.52; P<0.82).Conclusions:BMISR should not be treated as interchangeable with BMI M in BMI mortality analyses. Bias and inconsistency introduced by using BMISR in place of BMI M in BMI mortality estimation and hypothesis tests may account for important discrepancies in published findings. © 2011 Macmillan Publishers Limited All rights reserved.