OBJECTIVE: To assess the validity of self-reported body mass index in the National Health Survey. METHODS: 120 participants were selected and questioned about their weight and height with exactly the same questions that the National Health Survey uses. Afterwards, and once informed consent was obtained, participants were weighed and measured, and this data was used as the gold standard. RESULTS: On average, participants underestimated their weight by 1.39 kg, overestimated their height by 0.55 cm and underestimated their body mass index by 0.71 kg/m2. The sensitivity of self-reported self reported body mass index (BMI) to detect overweight was 77%, the specificity was 97%, the positive predictive value was 0.95 and the negative predictive value was 0.86. The Kappa index was 0.76 and the quadratically weighted Kappa index was 0.85. The correlation coefficient between self-reported and measured BMI was 0.96 and the intra-class correlation coefficient was 0.97. CONCLUSIONS: Self-reported data is an efficient way of obtaining information about BMI, although with limitations, because self-reported data tends to underestimate weight and overestimate height, thus underestimating BMI and the proportion of participants with elevated BMI.