Subjects were recruited from a previous survey, the Belen Health Study, conducted from July 1984 through November 1985. Based on the data of the earlier survey, 421 subjects were identified for recruitment and 275 actually participated. The Belen Health Study questionnaire included items directed at Sleep Apnea Syndrome. Subjects for the sleep study were selected based on responses to the initial questionnaire. For the sleep study, an additional questionnaire was used, as well as new techniques of home monitoring of breathing during sleep, and polysomnography to define the prevalence of symptoms and manifestations of Sleep Apnea Syndrome. An assessment of the health effect of Sleep Apnea Syndrome was made by reference to the demographic, socio-economic, and health data from the Belen Health Study. Diagnostic criteria and the utility of home monitors for clinical diagnosis were also determined.