The correlation between obesity and severity of obstructive sleep apnea (OSA) is well established in adults, but data are inconsistent in children. We hypothesized that there is a significant correlation between the degree of obesity and the severity of OSA in children. We retrospectively reviewed records of weight, height, history, and polysomnography of all 1-to 15-year-old children referred to our sleep laboratory. Children with known anomalies and repeated polysomnography were excluded from this study. Obesity was defined as body mass index z score (BMI Z score) > 1.96. The correlation between BMI Z score and apnea-hypopnea index (AHI) was assessed. Possible confounding factors, ie, age, gender, and tonsil size, were adjusted by multiple linear regression. Four hundred eighty-two children were included in this study. Obese children had a significantly higher AHI (median, 1.5; interquartile range IQR, 0.2 to 7.0) than the AHI of nonobese children (median, 0.7; IQR, 0.0 to 2.5). BMI Z score was significantly correlated with log-transformed AHI (LnAHI) r = 0.156, p = 0.003. BMI Z score and tonsil size were still correlated with Ln(AHI) even after adjusted for other confounding factors (p = 0.001). Degree of obesity as measured by BMI Z score and tonsil size are significantly related to severity of OSA as reflected by the AHI, although the correlation is mild.