For the prevalence study, 4,364 randomly-sampled men selected from the general population and stratified by age were evaluated for risk factors for sleep apnea. A total of 741 men found to be at risk were evaluated in the sleep laboratory to determine the degree, if any, of sleep apnea or sleep disordered breathing. Prior to recording subjects in the sleep laboratory, a thorough history was obtained from each subject and a physical examination completed. All men entering into the first four years of the prevalence portion of the study who were found to have sleep apnea or sleep disordered breathing were reevaluated yearly over a five year period in the natural history part of the study. The natural history portion also included 150 men previously studied in the sleep laboratory for apnea but who were not part of the random sample of 4,000 and who were not part of the prevalence study.