Several recent studies have demonstrated a positive association between obesity and asthma among women but not men. The present study examines the effect of misclassification of body mass index (BMI) in the association between obesity and asthma by gender. This cross-sectional analysis included a total sample of 961 Mexican adults. Use of measured BMI revealed that obesity (BMI >30 kg/m(2)) was a risk factor for asthma diagnosis in both men (OR, 2.5; 95% CI, 1.1-5.9) and women (OR, 2.3; 95% CI, 1.5-3.8). In contrast, use of self-reported BMI showed that only women (OR, 1.7; 95% CI, 1.1-2.7) and not men (OR, 1.3; 95% CI, 0.6-2.9) were at increased risk of asthma diagnosis. Use of self-reported BMI substantially underestimated the prevalence of obesity; this bias was not related to asthma per se but was mainly due to obesity. Therefore, misclassification of BMI obscured the relationship between obesity and asthma to a greater extent among men than among women since obesity prevalence in the general population was higher among men. Measurement bias merits greater attention in future research on obesity and asthma.