Although clinical experience has suggested for more than two decades that OSA is associated with impairment of cognition, emotional state, and quality of life and that treatment with nasal CPAP produces significant improvements in these areas, sound empirical evidence to support this view, especially regarding treatment outcome, has been lacking. More recent investigations have begun to provide this support from randomized, adequately controlled studies. These assessments suggest that some degree of cognitive dysfunction is associated with OSA. The effects are most apparent in the severe cases, whereas results in mild cases are more equivocal. Reported impairments include global intellectual dysfunction and deficits in vigilance, alertness, concentration, short- and long-term memory, and executive and motor function. Considerable discrepancy exists across studies with respect to type and degree of dysfunction, however. Disturbances in general intellectual function and executive function show strongest correlations with measures of hypoxemia. Not unexpectedly, alterations in vigilance, alertness, and, to some extent, memory seem to correlate more with measures of sleep disruption. Although many inadequately controlled investigations have demonstrated reversibility of most or all of these deficits with effective treatment, more recent placebo-controlled studies have raised doubts regarding whether the observed changes are truly a function of treatment. This issue requires further systematic exploration with adequate controls and step-wise analysis of treatment duration effects. A similar set of considerations exists with respect to the relationship between psychological disturbance, primarily depression, and OSA. Although several studies suggest significant depression in these patients, the results are mixed. Placebo-controlled treatment trials fail to demonstrate consistently a difference in mood improvement between active treatment groups and controls, although several methodologic considerations suggest that these results should be interpreted with caution. Numerous investigations leave little doubt about the issue of quality of life impairment among persons with OSA. Further characterization of impairment, particularly in areas specific to this population, will provide clearer understanding of the problem. Preliminary investigations of treatment response in controlled studies indicate significantly greater improvement of quality of life in response to CPAP. Although patients with OSA commonly report disturbances in cognitive and psychological function and general quality of life, the increased rates of obesity, hypertension, diabetes, cardiovascular disease, medication use, and related psychosocial complications present a host of potential etiologies that might explain the impairments noted. There can be little doubt that these covariants do, in some cases, contribute to neuropsychological dysfunctions. It is essential that future studies continue to define those disturbances that are specific to OSA, the relationship between levels of severity and impairment, the role of treatment in reversing these dysfunctions, and the correlation between test results and significant day-to-day social and occupational functional impairment.