Nasal continuous positive airway pressure (CPAP) therapy is in widespread use as the primary treatment for the obstructive sleep apnea syndrome (OSAS), a sleep-related breathing disorder affecting more than 15 million Americans. The therapeutic effectiveness of CPAP in providing significant, stable, and long-term neurocognitive or other functional benefits to patients with OSAS has not been systematically investigated. The revised proposed study is a randomized, blinded, sham-controlled, multi-center trial of CPAP therapy. The principal aims of the study are: 1) to assess the long-term effectiveness of CPAP therapy on neurocognitive function, mood, sleepiness, and quality of life by administering tests of these indices to subjects randomly assigned to active or sham CPAP; 2) to identify specific neurocognitive deficits associated with OSAS in a large, heterogeneous subject population; 3) to determine which deficits in neurocognitive function in OSAS subjects are reversible and most sensitive to the effects of CPAP; 4) to develop a composite multivariate outcome measure from the results of this study that can be used to assess the clinical effectiveness of CPAP in improving neurocognitive function, mood, sleepiness, and quality of life; and 5) to use functional magnetic resonance imaging to compare cortical activation before and after CPAP therapy, and to assess whether this change is associated with improvement in specific neurocognitive task performance. The primary endpoint of this proposed study is the effect of 6 months of CPAP treatment on neurocognitive function. A total of 1050 subjects (525 per treatment group) will be enrolled from the patient populations at five sites (Stanford; U of Arizona; Harvard; St. Luke's Hospital, MO; St. Mary's Hospital, WA). This study will advance our knowledge of the major, most debilitating, clinically relevant OSAS-associated conditions, and, by scientifically establishing the effectiveness of CPAP therapy, should greatly improve access for the countless victims now denied treatment.