<qa>  What is the primary endpoint of the study?The 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 the study is the effect of six months of CPAP treatment on neurocognitive function. A total of 1100 subjects 550 per treatment group will be enrolled from the patient populations at five sites Stanford University; University of Arizona; Brigham and Womens Hospital; Massachusetts; St. Lukes Hospital, Missouri; St. Mary Medical Center, Washington. 