It has been noted that sleep disordered breathing is associated with neurocognitive deficit, particularly of memory, learning, attention, hyperactivity, executive functioning, cognitive capacity, and poor school performance (Beebe 2006; Pandit and Fitzgerald 2012). And a number of studies have reported improved attention, executive functioning, analytical thinking, verbal functioning, memory and academic progress at 6-12 months post- adenotonsillectomy (Chervin, Ruzicka et al. 2006). In DS, study demonstrated that a higher number of apneic episodes on polysomnography was correlated to the decreased visuoperceptual skill in DS (Andreou, Galanopoulou et al. 2002). Similarly, presence of snoring in DS was associated with a much higher rate of disruptive school behavior than without snore (Carskadon, Pueschel et al. 1993). Although learning disability and memory defect had been globally known in DS, the behavior, cognitive, and developmental impairment caused by OSAS is especially concerning because it might adversely affect their ability, even the social adaptation (Rihtman, Tekuzener et al. 2010) . Therefore, investigators would like to know the correlation between severity of OSAS and Neurocognitive and behavior in DS people in Taiwan. Also, investigators would like to follow the Neurocognitive and behavior changes in those who had been treated for OSAS, including tonsillectomy or Bilevel Positive Airway Pressure.