The adenoid is a bacterial reservoir that contributes to chronic otolaryngologic infections. Staphylococcus aureus (S. aureus) is a common pathogen in the adenoid. The increase of antibiotic resistance in S. aureus has become an important issue in public health. The aim of this study was to compare adenoid hyperplasia and biofilm formation in children with S. aureus adenoiditis in Taiwan. The patients were divided into methicillin-resistant and methicillin-sensitive S. aureus groups according to the S. aureus obtained from adenoid tissue after antibiotic susceptibility testing. Adenoid hyperplasia was assessed by lateral cephalometry, and the severity of sinusitis was evaluated by Water's view. Microbiological investigation of available S. aureus isolates was performed by in vivo morphological observation and an in vitro bacterial biofilm assay. Sixty isolates of S. aureus were identified in 283 children (21.2%) after adenoidectomy, of which 21 (35%) were methicillin-resistant S. aureus (MRSA). The severity of adenoid hyperplasia and extensive biofilm formation were more prominent in patients infected with methicillin-resistant S. aureus than in those infected with methicillin-sensitive S. aureus (MSSA). The primary outcome of this study was to provide evidence that S. aureus constituted a significant portion of the adenoidal pathogens. The secondary outcome of this study was that MRSA adenoiditis may be associated with adenoid hyperplasia and biofilm formation.