BACKGROUND: Many children who have allergic rhinitis (AR) have adenoid hypertrophy (AH) which can cause significant morbidity. OBJECTIVE: To know the risk factors for AH in children with AR. METHOD: 208 children aged 3~15 years with AR who visited the allergy clinic of the Department of Pediatrics between January 2006 and December 2008 were enrolled. We measured the size of adenoids on lateral X-rays of the neck using the Fusioka's method and defined AH. Children were divided into 2 groups: those with AR and those without AR. The 2 groups were compared in terms of parameters, including obstructive sleep apnea (OSA) score, body mass index (BMI) and atopic charac teristics. RESULT: AH was observed in 52 (25.0%) of 208 children with AR. OSA score of >3.5 was observed in 19 (36.5%) of 52 children with AH. Obesity [odds ratio (OR)= 4.1; 95% confidence interval (CI) 1.8~9.2], nasal eosinophil counts (OR=2.0; 95% CI, 1.0~3.9), positive skin reactivity to molds (OR=4.1; 95% CI, 1.8~9.2), positive skin reactivity to Dermatophagoides farinae (Df) or Dermatophagoides pteronyssinus (Dp) (OR=2.6; 95% CI, 1.1~5.7) were significant risk factors for AH in AR children. CONCLUSION: Obesity, elevated nasal eosinophil counts, positive skin reactivity to molds and Dp or Df in children with AR may be associated with AH.