The neuropsychological performance, sleepiness and general functional status were evaluated in 330 subjects, including subjects with little apneic activity (RDI<5), mild activity (RDI 5-14), and moderate activity (RDI 15-25). Of these subjects, 75 percent were selected from a clinic-based sample, and 25 percent were recruited from an ongoing population-based study. 150 subjects with mild and moderate activity were randomized to receive 'conservative' medical therapy (CMT) or CMT plus nasal continuous positive air pressure (CPAP) therapy. Simple statistics, as well as multivariate techniques, were used to determine the relationship between SRRD (and associated hypoxemia, sleep fragmentation, and physiological sleepiness) to: a) intellectual abilities, attention and vigilance, psychomotor performance, learning and memory, and executive functions, and b) to general health status and function. Potential benefits of treatment specific for sleep apnea in subjects with mild and moderate SRRD were also determined after two months of CPAP therapy.