Background: Overweight and obesity are major public-health issues in Mexico. Sleep breathing disorders directly impact the health of affected subjects. The aim of this study was to describe the frequency and severity of obstructive sleep apnea/hypopnea syndrome (OSAHS) in Mexican adolescents grouped by weight status. Secondary objectives were to describe the influence of body mass index (BMI) and neck circumference (NC) as predictive factors of OSAH. Method: Somatometric, demographic data and full-night polysomnography were done. Linear and logistic regression models described the relationship between BMI and NC with OSAHS presence and severity. Receiver-Operating-Characteristics (ROC) curves showed different sensitivity and specificity performances of BMI and NC cutoff-values. Results: OSAHS frequency and severity were significantly greater in obese and extremely-obese adolescents (8% vs 22% vs 67%; p < 0.001). Linear regression showed an AHI increase of 0.940 events/hour for every centimeter-increase in NC and 0.696 events/hour for every1 kg/m²-increase in BMI. Logistic-regression showed extreme-obesity as the main risk factor (OR 9.9) for diagnosis of OSAHS. ROC curves showed an area under the curve (AUC) of 0.83 for BMI defined cutoff-values, and of 0.80 for BMI percentile-for-age cutoff-values. The AUC of NC ROC curve was 0.8043. Conclusion: This is the largest study made on Latin-American population evaluating the frequency and severity of OSAHS with full-night polysomnography. We showed that obese and extremely-obese Mexican-adolescents are at highest risk for OSAHS. Increasing BMI and NC showed positive correlation with AHI. Different BMI cut-off values showed similar diagnostic performance. Extreme-obesity increases 8 times the risk association for OSAHS.